One thing for sure: there is definitely something biologically different about them. They might look just like us even down to the cells, but their DNA (or whatever) evolved on their home planet. It has nothing to do with human DNA. A handshake will do, to get a few cells for analysis. You’ll see that even if they have DNA that uses all the same chemistry and base pairs, etc, it won’t be arranged in anything like the way ours is arranged. The layout of our DNA is very similar to that of chimps, because we’re related to them. The alien DNA would have a completely different layout, even if they were identical down to the mitochondria, because there’s no relationship between us and them.
But now that I think about it, if we already have aliens identical to us in every way except genetically, we might as well go all the way into fantasy and imagine that even the layout of their DNA is the same. Now that would be weird. They’d be humans, but in a weird way, right? Not related to us in any way, but we would be drawn to them the same way we’re drawn to each other. Yet at the same time we might be conflicted about how to treat them. They would seem strangely creepy (ok, maybe I’m the only one who thinks so); it would take a little while to get used to them.