There's a joke I've been meaning to write in a comic: An alien embassador talks to the president's house plant, then excuses itself for the mix-up: "You earthlings all look alike to me."
For that joke to work, the alien must be so different from anything we'd recognize as a living individual that it's very difficult to depict in a comic. Which is difficult to conceive. I mean, look at life on earth. Squids. Mold. Bacteria. Even worse: Viruses.They are so weird we're not even sure they are lifeforms in the strictest sense.
My aliens owuld have to be weirder than those.
All of the examples I've seen in comics and movies have been pretty much modeled after beings we know from earth. 'Cause where els ould we pull that from? But even our homegrown biodiversity is limited.
As for alien life, the common theories divide into two main threads:
1. Life began pretty much like ours anywhere it did with amino acids and stuff and evolved from there, most likely into carbon-based life forms. (The theory that life on earth was somehow transmitted from elsewhere is in line with this approach.) In this scenario, the first organisms will probably have been pretty much like ours, but the more it branched out, the more we must have grown apart, pretty much like we earthlings did except more radically because a) no actual common ancestors, just similar ones, and b) different environments, at least in details. Still, depending on when the different paths came into effect, there may be similarities like symmetric builds and the use of limbs and eyes.
2. It already started differently. Different chemicals under different circumstances took a leap nonetheless, developing at different speeds (I hear silicon-based lifeforms would be much slower in everything), evolving into something that is not life as we know it but has certain qualities that would be classified as similar to ours by an even more alien third intelligence. These beings could be like (or unlike) anything. They might not even be made of matter. Or information.
Either way, the difficult thing would be to recognize them. If they do the things we consider lively, like reproduce and self-preserve or even grow, eat and move, that's a pretty good hint. In those cases, we can move on to the next big hurdle, recognizing if they're intelligent or not. Which is even harder to tell if we have no way of communicating. If they don't do any of those things, though, or if they evolved to be only visible in the spectrum we're not equipped to see or if they look like rocks to us ando only if you see infrared you see their movement or if they're entirely made up of smells rather than biomatter or if they're just reeeeeeally slow, one heartbeat in meillenia slow - in those cases we may have absolutely no way of knowing if the thing we're looking at is alive or not, or even if we're looking at something at all.
