How can you tell if a guy will let you blow him

Yeah, I tend to agree with rltg4711, just ask him if he'd be interested in a BJ. I mean most, if not every guy out there would accept one.

Yet again, (if you are by chance a guy, [Been there, done that myself],) looking to investigate/experiment with things of that nature, then I would recommend staying away from friends, or people you know/knew beforehand...

Get em a little tipsy then ask - will increase your chances by 50%. "The drunker the guy the more he is bi."

You could simply ask, yet that just may blow (pardon the pun) all your chances. Simply be friendly in an honest way and who knows where a conversation may lead to and perhaps the oportunity for you to be a littled more straight forward drop a couple of subtle hints. You can sort of gently reach out and touch him during the converstion, just like long-term friends do. If he accepts being touched, perhaps on the arm , then at least you know he accepts you. Take it from there.

Just ask, the first blow job I gave was to a stranger, I just asked if he wanted head, he said yes, moments later my mouth was full of cock

Just get him horny and he will love it...

He'll whip it out for ya