To be honest, even though a guy may think being in the "friend zone" is a second rate relationship, I've found that the depth of my friendship with guys who are in my friend zone is just incredible - we can talk about anything, share anything, go skinny dipping, talk about lovers, etc -- they are truly friends, and I know in my heart that some of these guys I will know and enjoy as friends when I'm 75 years old.
I've only brought a couple of friends out of the zone - and knowing its time is simple: I'd do something with them I'd never done, like a very deep French kiss, a very deliberate and sensual cock rub on the pants, anything like that that normally doesn't happen between "just friends"...
But be careful - it changes the whole dynamic: I'm no longer a good or best friend, I'm a girlfriend he has sex with, and may end up getting jealous of time I spend with other guys. And he is no longer a really great guy friend I can talk to - he's fucking me, I'm sucking him off, we're enjoying each other in a heavenly way ---- but, unless the committment is lifelong, boyfriends and girlfriends inevitable fall apart and break up.
And, once broken, it might be impossible to get back into the friend zone.
Personal experience: I've got about 7 guys in the friend zone - they all know each other, and we are all friends as a group. I had brief but intense flings with two of them, bringing them up into the sex zone. Both flings were great, but one ended as flings usually do with jealousy over time or someone else, or someone else comes along. The one with that guy actually put me and the guy into a vary different zone - we moved down from friend zone to just cordial after the breakup - we say hi but never do much more than that.
With the other guy, we managed to see what was happening and decided that our friendship had lasted for such a long time pre-fling, and we wanted it to be that way again. He's back in a "modified friend zone" - since we do know each other intimately, every now and then we'll "get together", but its no where near a FWB level - he's not a booty call for me, and I'm not one for him. If we find ourselves together somewhere for some reason, and there's not much else to do, we find something to do. But we're very careful. Getting back to that "friends until we're 75 " thing - when I look at him I know I want to have sex with him when we're older and hope we can at 75, but we'll just continue to be the best of friends.
Confusing I know - and probably playing with dynamite. But he's a friend I want to keep, and even though I enjoy sex with him when it happens, I enjoy all the other times of just being best friends even more.