I noticed when I found a vodka receipt in his car (with an explanation of course). Then nothing for a couple of months. Then I started smelling it on him and various other signs. It progressed and became blaringly obvious in about 4 months.
After he admitted it come to find out it had been going on much longer than I knew (the initial vodka receipt - that was in May of last year). He had been dabbling since January.
We live in close quarters, spend a hell of a lot of time together and I had no idea. He is also a poor liar but not so much when it came to this.