He cares about personal hygiene/grooms himself/is not dirty He wears nice clothes He owns a pink shirt He doesn't play sports He's fashionable He's tidy with his stuff He ate a salad once He is "out of your league" and isn't interested in you He watches X He listens to Y He says Z Other ridiculous bullshit
Alright you get the idea. For the record, much of the above list might be more accurately be "clues your friend is European". And before my list, let me go ahead and admit it's not going to be perfect or applicable to everyone, but based on my experience, it's a more accurate compilation of clues that your friend is not a hetero. It's also less about any individual clue than many in conjunction.