Excerpt:

Sirius could not remember the day he had discovered that Remus was a werewolf. This was largely due to the fact that no such day existed. It has been at first a niggling sense of 'something's not right with his stories' and 'he's keeping a secret from us.' The list of plausible secrets had eventually shrunk under Sirius' and James' dutiful determination. Remus had once supposed that they were so dogged about discovering his secret because they fancied him a good friend, but alas, such was not the case. For the likes of Sirius and James, secrets were simply meant to be found out, the way puzzles were meant to be solved, snowballs meant to be thrown, and Slytherins meant to be hexed. The function of secrets in the world was to provide mental stimulation for bored Gryffindor boys who hated classes. As secrets went, Remus' was not kept with as much diligence as one might expect. He'd lived mostly in isolation and with his parents. His excuses and stories fell flat, and later James would remark that he hadn't really made an effort on them if he'd been hoping to waylay the ilk of Sirius and James.

The result of these facts was that Sirius had never had a moment, nor even a day, that contained a 'Aha that quiet bloke we share a room with must be a werewolf' moment. Rather, it was a slow realization. Once he had acknowledged the high possibility of the unlikely scenario that the boy in the next bunk over was a dangerous and dark magical creature, he easily could see that he'd more or less known the truth if the matter for a good week or two and had simply been coming to terms with it before he could say so to himself.