Ummm....

[Okay she is seriously struggling to think of how to spot this. It's so much easier with wolves. But she is going to take a blind stab at it anyway.]

Well, there's the way people emit pheromones, and that's really the most reliable way for me to tell I guess. There's also body language. If they're interested, they'll focus more on the person they like even if it might not seem that way from what they say.