I am confused and don't know where to go for help. I can't tell if 15 yr. old is drinking or smoking pot. How do you know? What is the morning after like?
Play it cool, or let 'em have it? Appropriate first offense responses?

if they are going to do it.. they are going to do it. wether you like it or not. But do tell them the consequences of what they are doing. and maybe there might be a chance that they will stop or make the choice of not even starting.
I had always snuck behind my mother all the time. But if she didn't give me a chnace to do it... i didn't do it. For instance... I had a curfew of RIGHT after shcool to be home. I could have friends over but if i wanted to go to friends house, my mother had to KNOW who they were. and if she didn't like them i would not go. I hated it all my life becuase of the tough rules. but i am also VERY VERY grateful to her. I could have been a drug addict or dead somewhere. I never really started anything till i was around 19 to 21. And by then you saw what it did to you.
Make them get a job and pay for room and board with you. Anything to keep them busy.

To be able to tell:
Pot - they usually are tired all the time. They will munch on food a lot. They become lazy. Other things.. if you can't smell it on them... or if they smoke <that makes it hard> Or if they use patchuli scences <strong perfume>.
Drinking - Just like you! a headache in the morning.. you ask them to do something like clean and they claim they feel sick or not good to do it.Are very quiet all day long <hint: not playing that music loud that they usually do>

Chances are... you even THINK they are doing it.. THEY ARE.
Look in their rooms. Look through their stuff. Its not wrong of you to do that. Look under the mattress in socks.... boxs.. even the smallest. You find something confront them. THey will NOT hate you forever... <maybe for sometime> but not forever!
I love my mother for it. She has made me the person i am today.

let them have it! If you play it cool they think they got away with it and will keep on doing it. With pot their eyes are red and droopy and they can't look you in the eye. In the morning I never really had any symptoms except my clothes smelt like burned leaves. Drinking is in their breath they clothes their disposition I'm with jennifer go through their stuff they'll only hate you till they turn 21! Stay strong it doesn't last FOREVER.

PS FIRST RESPONCE talk to them first tell them you know whats up and if you see it again ALL HELL WILL BREAK LOOSE. First look then ask becuase if you ask first they'll find a better harder place to find it. (believe me expeienced hellraiser)I was a really bad kid. But I was a girl. It really depends on his own morality and sadly how much he fears you and you aproval of him. If you know you son is the one to act before thinking tough love is the only way. If he's a good kid AND YOU KNOW HE'S A GOOD KID(don't kid youself) talk to him let him listen to reason.