I must be in the minority, because I think snooping is very wrong.
TCG, I found out my son smoked weed at 14 because he wasn't home when he was supposed to be and there was a letter on his bed. I thought the letter was meant for me, but it turned out it was from his girlfriend making jokes about how VERY much he liked his weed.
I made a big issue about it, we both went through a 6-week programs, I had him tested on a regular basis, etc. etc. and today at 25 he is more lost than he ever was before. I often wonder what would have happened if I never read that letter, whether it would have been a phase as opposed to the defiant power struggle that ensued for many years thereafter. So I guess we all wonder what we could have done differently for a different outcome.
I think you have nothing to gain by snooping. I personally think it is too late to change your child. They are formed and whilst you can try and gently influence them, they are in a rebellious stage so how much you can achieve is questionable.
If your son finds out you are snooping, he would be very angry and I think rightfully so. My very normal and diligent 15-year old will never forgive me if I read his texts. We do have to respect their privacy where drugs are NOT concerned.
I think you can drive yourself crazy snooping. Just my opinion.