How?

first signs are a bloody discharge and swelling of the vulva, soon followed by hormones making them act different and other dogs around them paying more attention to them. Some dogs show very little signs, and can be difficult to determine if living outdoors on dirt. My terriers have to use what is call the paper towel test because they don't swell either. I would wipe there little Vulva with a white paper towel everday when it was getting near time for heat cycle. to look for a slight bloody tinge.
If you miss the cycle you are going to get male dogs showing up in your yard from miles around trying to breed.