Diverse or Not?

I have several very attractive female workers in my Starbucks. We serve lots of men each day in our store. We have a problem and need some tips.
How can you tell if a guy is diverse/gay?
We have a bunch of good looking guys who come in each day and we are wondering if the are gay. Short of asking, how can you tell?
They look good enough to be male models and wear all the hip clothes but never come in with girls.
They will stay around an talk to us but it never goes any further.
I want to help my friends out here.
We used to have a barista who had excellent Gadar but he moved to San Franciso. What a loss, he was a great guy!
So tell us how can we tell???
Coco