I think the most revealing clue is his anger when confronted. You don't really need any other clues. If he was innocent he would be understanding.
When my son relapsed it was after his developing kidney stones and the doctor prescribed pain meds. I noticed that he was really out of it and I asked him "How many of those Percocets did you take?" He was supposed to take one three times a day. He got all bent out of shape "How can you say that to me, Mom? Do you know how that makes me feel?" Red flag. He even called his girlfriend and complained to her about my comment. Later, when the truth was revealed to me he was abusing the pain meds (taking 20 per day rather than the prescribed 3) I thought to myself how upset he got that I was on to him. I truly believe had he been taking his meds appropriately he for one wouldn't have been so out of it and even if he was he wouldn't have been upset with me. The same with your husband.