When he's sucking you off like a freakin hoover
Seriously? I suppose if you are alone together and he starts asking questions related to guy on guy sex I'd say he's curious. A lot of people believe in "gaydar" but I'm iffy about that. Sometimes straight guys with no intention of sex will check you out so it's hard to say that a "look" is an indication. Speaking from past experience if a so called straight guy is curious some how he will let you know without you having to ask. In fact don't ask. If he's gonna cross the line let him walk over it all on his own.