I'm going to link you to a page that answers your question for you.
v
Official Boozeception GUIDE
Thanks, but it still doesn't answer the question of when you can tell that it's fermenting.
Also I can't seem to tap the barrel... Apparently I was clicking the barrel wrong with the tap 
Hi
I've scoured the forum but can't find my answer.
How do you know when the booze is fermenting? Currently I am trying to make rum with reeds and beer with water cells and wheat. I've tried once before to make this and waited three Minecraft days before first trying to tap it with the tree tap then with the stone mug. Am I missing something obvious?
If it's relevant; I'm doing this on a Tekkit multiplayer server.
