That is a good question. Sometimes, without a stallion to 'tease', mares will either not come into heat or have a 'silent' heat. It may take someone to 'crank her up' to show that she is cycling. If she is not showing, do not worry about it. If you were going to take her to be bred, I am sure she would show to a stallion when it was time.
I have 5 mares and no stallion and I know when 4 out of the 5 are in heat. When my 'silent' mare went to be bred she was not silent anymore...LOL. Most of mine I can tell by the way they hold their tail.
We have a few mares in the batch that tell us by screaming and mounting the mares in heat and right now they are making a big fuss over the oldest one out there who's 27 now but we don't breed her anymore. And l noticed this morning she stands and winks and pees for them so looks like a good heat anyway.
Our old horse that we had (But sold a couple month ago) was the only mare we ever had. She wasnt a mini (16.4HH). But we had her in a stall next to our stallion and she came in heat im sure but we never really noticed a differense. I still road her daily usually. I guess it varies from personality to personality.