My boyfriend is on methadone but i fear he could possibly could be using?

My boyfriend is on methadone (so he says) but when he gets home from the clinic he's always really tired and goes to sleep. Lately I've noticed the inside of his nose to be brownish. He usually should only take 40 min. Or so getting dosed but that ranges to about an hour or two I don't see track marks but he says he goes to work but his pay stubs don't match he just acts funny sometimes I just need some answeres about if he could be using.... no advice please... thank you!