I don't like to be "that" guy, but you really have to research this section of the forums (drug testing). All of the answers you are looking for are in here, I know because I've freakin read waaay too many of them.
They test for creatine levels because a low level indicates a dilluted sample. Read the stickies and you'll know why.
B12(plus the B complex) add color and some weight to the urine, that's all.
They also test for salts in the urine, those add to the weight.