If extraterrestrials look like us, then how can we tell them apart from us?
That may be difficult. I know that during the biblical times, Yahweh freaked out and killed the brother of Aaron, when he shined a purple light near him. So it may give us a clue that certain frequencies of light, near the ultra-violet band, might be able to point them out to us. I am not positive that that method will work with all of the groups or even with the current crop, but we will find other ways by asking the friendly group what they suggest.