For many people, the often outlandish proposals and predictions of futurists are just obviously impractical and are to be laughed off. This attitude, irrational is it may seem to futurists of the stripe who take outlandish ideas very seriously, is itself not to be sneered at -- automatic unbelievers in the alien save themselves from chasing many red herrings. Those who laugh at futurism because they are unimaginative dolts I will not try to defend, but those who laugh at futurism when futurists take themselves too seriously are usually spot-on. For those of a more serious nature and intellect who want to actually figure out the flaws in futuristic ideas, here are some heuristics:
(1) Find the easier thing. If there is an easier way to get much of the value from a proposal, ask yourself, why hasn't somebody pursued this easier way? For example, seasteading proposes the creation of novel structures for people to settle permanentantly in the ocean. Ask yourself, why don't there already exist communities that live permanently on cruise ships? Why haven't oil companies moved the families of their offshore platform workers out to live where the work is?
(2) Look to see if if the futurists have proposed experiments that can be done much sooner and more cheaply that would verify or falsify the propsosal or prediction. Many of the "most important", in terms of perceived future impact, hyper-futuristic ideas are conveniently unfalsifiable: artificial intelligence, uploading of consciousness, and so on. There are a near-infinite number of unfalsifiable theories that our imaginations could dream up, making the odds of any given such theory to be true about zero. The ability to conduct such dispositive experiments, the ability to prove a hypothesized event false if certain conditions occur, paradoxically makes that event far more likely. A related heuristic is to be very leery of ideas that, as is said of fusion power, are "always thirty years in the future". If the futurist can't explain why the futurist of 30 years ago who predicted something similar was wrong, that futurist should indeed be laughed at, early and often. Far too many futurists are so futuristic that they know little about the past which they purport to be projecting. Some don't even know when predictions similar to theirs were already made decades ago, and were already supposed to have come true. At the same time, be wary of futurists who are not willing to make short-term predictions, lest we obtain a track record of the vast uncertainty involved in their brand of futurism.
(3) Except for rare phenomena of high predictability, such as the orbits of planets, past performance does not guarantee future results. Futurists often chart exponential curves of growth in some measure of technology: the speed of transport, the number of transistors that can fit on a chip, and so on. The first half of a logistics curve looks much like an exponential curve. You can fit an exponential curve to the data points, but it's really a logistic curve, which in the long run, and possibly even in the short run, will lead to a radically different kind of future. Because of physical limits and human psychology, reality far more closely resembles logistic curves than exponential ones. For example, world population growth seemed to follow an exponential curve until about the 1960s, when it flipped into a quite different mathematical regime. This transition to sub-exponential growth started much sooner in the developed world, which should have been but was not a clue for the population alarmists. As for physical limits, a good example is transport speed: it seemed to be growing exponentially until it hit the sound barrier in earth's atmosphere and the implacable nature of earth's gravity well beyond it in the latter half of the twentieth century. More on the dubious nature of exponential projections here.
(4) Beware of the prophets of false certainty. These are people who focus on one out of many possible outcomes, or take very seriously unfalsifiable predictions, or follow exponential projections, or have neglected to find the easier thing, and pretend, because nobody has proven them wrong, that their version of the future has a high probability. We have, for example, the Bayesiologists, who, while to their credit are at least aware of first-order uncertainty (known unknowns), neglect the higher-order uncertainties (unknown unknowns) inherent in most futurism and demand that we make some intuitive guess as to the numerical probability of their predicted event. (When asked for an intuitive numerical guess about some hyper-futuristic prediction, "50%, plus or minus 50%, distribution function unknown" is usually the best answer).
(5) Look at interests. You may not understand the science involved, but individual and institutional interests are human universals. Take astrobiology, for example. Here we have a science without a subject. Now the astrobiologists to a man argue that extraterrestrial life must be common, indeed that it may well be right around the corner underneath the ice of Enceladus or Europa or on one of those exciting new exoplanets. There appears to be, as many activists like to say about global warming, a "consensus" among the astrobiologists about the ubiquity of life in the universe. But only primitive life, of course -- otherwise the uncomfortable fact that we have never observed the signs of any artificial surfaces, despite observing billions of stars in our own galaxy and billions of other galaxies, would rear its inconvenient head. Thus the Rare Earth Hypothesis, in which for clever reasons life is supposed to almost always stops evolving beyond some primitive stage, in sharp contrast to the ongoing evolution of life to higher complexity in the only history of life we have actually observed. Does the astrobiologists' consensus reflect their expertise and your ignorance in astronomical and biological matters, or does it reflect something else? Consider this -- if you were skeptical about this astrobiological thesis, why would you become an astrobiologist in the first place, risking your career on a science that has no subject? If the politicians and academic boards who fund them ever became convinced that extraterrestrial life probably does not exist anywhere where we will be able to observe it before they retire, astrobiologists would have to find new jobs. This is a career for true believers. Beyond this rather dramatic selection effect, we have individual and institutional self-interest to keep the argument going -- to fund their careers, astrobiologists must persuade us that life in universe is common, common enough that we should fund multibillion dollar telescopes and spacecraft and, of course, grant copious amounts of research funding to them in order to look for it within or astronomically very near our solar system, which is as far as we can observe the signs of primitive life. Even if you know nothing whatsoever about either astronomy or biology, but do understand a thing or to about humans, you are wise to be highly skeptical of the claims of astrobiologists.
(6) Be especially skeptical of political futurism. From NASA's Shuttle and Space Station, which were supposed to revolutionize space industry, to the politicization of doom-and-gloom scenarios such as overpopulation and the supposed dire consequences of global warming, politics mixed with futurism has a very poor track record. By contrast, private entities like the Singularity Institute, Foresight Institute, and so forth, while even more outlandish and preposterously self-serious, can provide creative starting points for brainstorming towards more practical ideas and are relatively harmless.
(That leads me to my last heuristic -- (7) avoid futurists who can't laugh at themselves).
Futurism at its best is a creative and entertaining game of ideas. Playing with outlandish ideas can be very productive -- for example, the Easier Thing on occasion may turn out to actually be a good idea you can implement now, and you arrive at the easier thing by starting with an outlandish idea. I occasionally explore outlandish futuristic ideas here at Unenumerated, which prides itself on an unending variety of topics. There is nothing to sneer at about futurism as fun unless you have an unimaginative rock for a brain. However, those who take these ideas too seriously, or have created a false sense of certainty about them, do deserve a few guffaws.