The 1st thing you should do is ask him. If your not satisfied with his answer then go on to what Ima said.
The day he got out of the drug rehab he should of hit 90 meetings in 90 days......
The sure fire way of telling if somene is on cocaine is checking his pulse. If it's racing, then most likely yes, he's on it! Cuddle up to him and put your hand over his heart! Also, dilated pupils usually too.
I hope that helps! Keep us posted!