If there were extra-terrestrial aliens living among us, is there a sure-fire way to determine that?

They wouldn’t get the “in” jokes of the nation. “No soup for you!” Going commando.
The theme song (or actually, whistle tune) from Andy Griffith.

Maybe we wouldn’t. If they had managed to develop the ability to mimic humans on an absolute physical scale, perhaps they would have also studied cultural markers. So many of our shared references are spread by universally accessible media. Or they could pretend to be from another Terran culture.

Make ‘m cry.

When they burst out of your chest!
Or when they have a congressional hearing.

Probably. But it would be a violation of every Earthling’s privacy to find out.

You could send a DNA sample in to 23 and me or similar service.
There wouldn’t be an ancestry line or other relatives.

Nothing biologically different? Doubt if there will be a sure-fire way to tell them apart then. If they start swallowing small live reptiles or voting for trump, it wouldn’t raise alien—alarms.

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.

I would think they wouldn’t know how to laugh at things we human beings laugh at. The Data test.

There is no sure fire way but anyone wearing a baseball cap with the letters MAGA on it should immediately be sent off world as a precaution.

^^ lol, good luck with that. ^^

Suppose their DNA did match, in a very ancient, pre bottleneck way? Do we welcome them as long lost family?

Or, suppose we like them better than our own human race. Do we put them in power of our populace?

@LuckyGuy – 23andMe would find that the number of chromosomes is not equal to 23.

They would be driving a Meteorite!

Ask Lois Lane?