Is there anything else that would indicate drug use besides the teeth? How bad are the teeth?
If there are other things, you might just have to be forward and direct. You might say "I've noticed some signs that you might be getting into drugs and I'm worried." simple as that. Then, let him go from there and he'll either reassure you or get paranoid and angry (meaning yes, he's on drugs) If he frowns, looks around scared-like and says "Who told you that?" in a paranoid voice....better keep your daughter away from him and insist on drug testing before visitation.
That's some serious stuff. I'd start doing some research on it. You might even call the cop brother and say "look, I'm worried about such and such because of what other people have been telling me. Can you look into it before it gets serious? I didn't know who to talk to about it, but I think I can trust you to find out if he's ok" If he cares for his brother, he'll want to make sure he gets to his brother and gets it taken care of.