It depends how organised they are. I always know from my diary when my 28th day is, but usually I find myself feeling spotty and emotional, and then go "OH!!!!! I remember why I feel like this!!" and suddenly everything makes sense. My sister on the other hand becomes a horrid nagging short tempered cow-bag, and NEVER knows when she is about to get her period. I ask her if she is hormonal and she says no idea- and 2 days later she gets her period and all is explained. So, I think the answer is no; women don't realise that they have turned into the she-devil, at least- not straightaway.