Signs

I new to all of this. So please don't be mad if this doesn't sound right.But I was at a store getting some stuff printed and this cute cashier helped me out. Now I can't tell if he's gay or straight. I got kind of a gay vibe from him with mild mannerisms and his voice. But anyway how do you guys know if a guy is gay or straight. I would ask but I feel like it's breaching someone's privacy.