^It's a little thing called "science." Understanding physics, mechanics, and biology can help you figure out a lot about what might be possible, what's unlikely, and what's definitely impossible. For instance, a human-sized alien with insectlike compound eyes is implausible because compound eyes have low resolution and are only practical on the very small scale of insects; something like the Than in Andromeda or the Tritovores in Doctor Who: "Planet of the Dead" should by all rights be effectively blind. A species with crablike pincers instead of fingers or analogous manipulative organs wouldn't be capable of complex tool use and thus couldn't make clothes or machines or starships. A species capable of flight would have numerous aerodynamic and structural constraints on its anatomy that anyone familiar with the mechanics of flight can deduce, though of course it's dependent on the density of their native atmosphere. (Not gravity, since gravity and buoyancy cancel out precisely.)
There are scientists and SF writers who devote a lot of thought and care to figuring these things out. I've spent the better part of my life working out scientifically based designs for alien species, even going so far as to work out their evolutionary history. Of course it's speculation, but it's informed speculation.
