Smiling&eye contact- what separates them just being polite/being friendly and them being attracted/interested in you? What’s the sign it’s ok that you go talk to them?

I think it’s pretty normal when someone smiles at you, you smile back (even strangers)

but what makes it different when they are genuinely attracted to you/interested in you?

esp when you see that person at the building you both live in, or when you are at the same gym?