Alien samples won’t have carbon-14, which is created in the atmosphere.

You’d sequence this organism. If it came from earth you could tell. People would have either sequenced it already or a distant relative species where you could compare sequence divergence and when they shared a last common ancestor. Convergent evolution might lead to similar phenotypes but the actual sequence of genes involved is only going to be similar if those genes shared an evolutionary history.

Chemical space is very very large. Like we can say for certain that only a very tiny percentage of all possible synthesizable organic molecules have ever existed in our universe. The combinatoric explosion is just so enormous that I think its incredibly unlikely that the only configuration of molecules that supports life is our own here on Earth. In fact, I think we know this given that there are artificial amino acids that can be incorporated into our existing protein biochemistry without a problem. Thats a small example but it points to a much larger space of chemical compositions that can support life. So an entirely separate evolutionary process would almost certainly land in a very different chemistry for things like information storage and molecular machines like protein.

Also our biochemistry is compositional, it takes small building blocks and remixes and combines them to build larger structures. I suspect that this is also a necessary feature of life in general. It’s very hard, basically impossible, for natural evolution to build huge structures like proteins just by pure uniform random selection. Instead it takes small pieces randomly, then puts them together to get complex life.

Point being, its a very very path dependent process. Any small difference in the early building blocks gets exponentially magnified when evolution uses those blocks to build life. So that leads to easily detectable, drastic differences in biochemical structure. This is evident on earth in that our biochemistry has a feature called left handed chirality that seems to be a purely random accident of the very earliest steps of life. That then was transmitted to every living being on earth. There’s probably a fifty-fifty chance that extraterrestrial biology is right handed instead. Every step in evolution also probably has random accidents just like that. Our particular biochemistry is the result of a trillion coin flips. There are probably many other biochemistries that work just fine, but look way different

> How would we tell?

You'd send a second mission to do analysis in situ.