Spaying (and neutering) is best done at 5-6 months of age. A full exam needs to be done, as well as testing for feline leukemia and feline 'AIDS'(FIV). Also, a stool sample needs to be tested for parasites.
At this age, most cats will not have been in heat yet - spaying during heat can cause greater chances of complications for the kitty and difficulties for the surgeon. Cats don't have 'periods - they stay in heat until being bred or manually stimulated to temporarily go out of heat.
Signs of heat include "acting slutty" - yowling, rolling at the owner's feet, presenting her back-end, rolling around on the floor.
Be a responsible pet owner, don't put off spaying/neutering as shelters are filled - and have to kill - unwanted pets due to 'accidental' or 'breed them once' litters of dogs/cats.