Mares are only receptive to the stallion when in heat - they are not only uninterested in sex the rest of the time but actively averse and likely to attack their potential mate.
Because of this, it's important to be able to tell when they're in season. (Being in season can also affect a mare's willingness to work - either because she's distracted by being horny or, in some cases, experiencing physical symptoms such as cramps).
Some mares are more obvious about being in heat than others.
A mare who is in season will hold her tail in an elevated position and do what we call "showing" - the tail is lifted to one side and the lips of the vulva will open and close in a rhythmic pattern. In heat mares will show not just to stallions but to geldings. Some mares will even show to a human handler. (Yes, horses will flirt with you). They may also appear to be distracted when working, and some mares find it almost impossible to concentrate when in heat (They're horny. Really horny).
The other obvious symptom is urinating more frequently. Horses normally only urinate once or twice a day - an in-heat mare may urinate several times a day, but not produce much urine. Moving into a urination stance but not urinating can also be a sign of being in season.