Just look at your males. If she is in heat the males are easier to spot
That is how I know when a female is coming in. My males go crazy and annoy me. I see they are ready for action so I start lifting tails to see which females are coming into heat. I did have a gyp have her first heat and stayed in heat over a month so we went to the vet and got some med to end it. But she was still bleeding. She has normal heat cycles now. But they can have hormone issues.