Its difficult to tell but as an initial guide expect it any time from about 6 mths. They usually have some sort of mood change, so more clingy, whingy, snappy, lethargic than usual or as if in discomfort. You may see a regression in toilet training, ie waking up to urine accidents when they didn't happen before or notice more weeing during the day. Don't scold for any regression as it can't be helped during this time. You may also notice that she starts licking herself quite a lot between her legs, usually because the blood is already flowing. You also may simple notice small spots of blood on the floor or in her basket. I'd also say to make it easier to check her bedding lay down a light coloured towel or blanket over her basket so any blood will be clearly seen. You could also try dabbing her vulva gently with a white piece of tissue paper just to make sure you're not missing anything.
