I remember asking a ex-fast jet pilot turned airline pilot the same question when I was 15. The answer I got.."just apply - they'll soon tell you mate".
I was a flight seargent in the ATC, then joined the UAS at Uni, and one thing I can tell you is that you are not 100% sure you want to join the RAF you don't stand a chance, as I think WWW has said, make no mistake - they will sus out your lack of commitment at one stage or another. It costs millions to train RAF pilots and defence budgets are shrinking rapidly.
However, I would disagree with the "best of the best of the best" comments. Its too ambiguous. The RAF are looking for certain personality types with a certain level aptitude for the many months of hard officer and flying training ahead. The current demand and supply of candidates also play a big part obviously. The decisions as to who is offered a commission are, at the end of the day, made by humans and their idea of "best" is very specific to the RAF. Someone with great grades, fantastic flying aptitude, etc etc will fail to get it in if there is only 1 place up for grabs and they feel another guy in the selection group is a better teamplayer/potential officer...for example.
Find out about what life in the RAF is like today (military forum on pprune a good start) and think hard about life in the armed forces is for you. Also consider how you feel about the possibility of serving in a conflict such as Iraq? The RAF is obviously not a flying club!
And finally, dont listen to anyone who tells you "you're not good enough", especially yourself! Just apply and don't fear rejection nor take it personally if it happens. Its a big world with endless opportunities!
All the best anyway dude.