I can think of one (semi) recent time where I confronted him. He was slurring his words and falling all over. He's diabetic, and fervently denied drinking.
I simply announced that if he wasn't drunk, he was clearly having some sort of metabolic issue, and needed to be evaluated by a doctor (other than me.) So I took him to the ER. He started confessing halfway there. Two beers became 6, and it became painfully clear to me that yes, he'd relapsed.
He ended up going back to rehab and I left him shortly therafter. I don't know if that really applies to your situation, but it was a turning point for me.