Aliens glow when they enter a "very" emotional state, e.g. very flirty (they glow pink) or very sad (they glow blue).
I don't think my scientists have discovered aliens just by seeing them glow, but usually while chatting with them, they can randomly use their alien brain powers on you and that usually gives my sim the "xenophilia" confident moodlet or the angry moodlet of the same thing.
My other sims have also discovered aliens by WooHooing them, because they can't hide anything during WooHoo 😉
I haven't done much analysing the DNA samples yet, but I think you just have to keep trying with those, as I've mostly just discovered traits using DNA analysis, but I assume there's some chance of detecting alien DNA?