Well it's pretty easy to tell in the early days, you'll start fretting notes without a buzzing noise, it won't take you 30 seconds to fret a chord before you can strum it, and so forth. But what about when you are past all that, you can play a riff or a song, and it felt good. Was it good? Was it in fact fantastic, or was it in fact pretty poor?
First up, especially for my mate Oliver, I've got to say "it doesn't matter, as long as it felt good to you." That's the personal triumph, the win over the challenge. You gave yourself some joy and goodness knows we all need some joy.
But I believe part of getting better is throwing yourself to the wolves, and seeing if they eat you up, or you eat them up. Josh (thanks for the tip Rom, he's an excellent read) would say "Why are guitarists such haters?" and I echo the sentiment. So if you do throw yourself to wolves, be prepared for unconstructive criticism.
And it's hard to take criticism for something that is an outpouring of *you*, it is your emotion, your feelings coming out on the guitar. You are digging deep when you play the guitar to others, you've got your heart on your sleeve...so criticism will equally dig deep. Look, I'll just say it: you're gonna have to get over it. Within that criticism may be positive stuff you can take away with you and learn from, the rest, you are going to have to just let it go. Ignore the negative destructiveness - or stick to the comfort and security of playing purely for your own personal joy.
For me, I was already thick skinned before I started putting stuff out there, thanks to the internet. I had run various site messageboards and often got a "You are an idiot and your stuff is stupid". At first it would ruin my day, and regardless of how many people would rush to my rescue "don't listen to that guy JAW" it was always that knife that stuck, the 99 other positive comments didn't pull it out. Several years before it ceased to have an effect on me.
With youtube I had to get even thicker skinned, there is even more "you" being put up for potential ridicule. I think as you get older you get less sensitive. Thank goodness!
So jump in, put something out there, be upset by haters, then let it go, and in between get some potentially useful feedback. For example, on youtube you might get "Your playing was out of tune" - relisten to it; hey yeah, I never really noticed. I must take more care to correctly tune up. "Your tempo sucks" - relisten to it; I *was* playing fast, and speeding up as I went! I must get over that performance anxiety, and must practice playing against a metronome. "Ha, your face looks funny" - yeah okay, I have guitar face, and you know what - I don't care :)