Some Invaders could be identified by what David Vincent calls a "deformed fourth finger". Technically, the finger is not deformed, but immobile, always sticking out. There are hints dropped from the beginning that the aliens are correcting this problem so that not all invaders have it. Toward the end of the second year they seem to have eliminated the defect altogether.
The true form of the Invaders was never completely revealed, although there is a hint of it in episode 2.7, "The Spores". Their human forms were unstable and had to be "regenerated" periodically, or else they would die.