Being able to determine if a sow is in heat is very important to your daily health checks. Sows will cycle every 15-19 days on average, although some will have a slightly longer or shorter cycle than this. A sow will begin to cycle around 24 days of age, and is sexually viable at this time. For this reason, I advise removing baby boars and weaning them at 3.5 weeks of age, even 3 weeks of age. Dad usually accepts them with no problem and it keeps mom and sisters safe.
Being able to track a sow's cycle is important for many reasons. For the first time gpig owner, it's a very good indication that you have purchased a pregnant piggy if she doesn't cycle in the first month. If you are trying to breed her, it's important to know when she is cycling so you can put her with the boar during that critical time frame. This is especially important for breeders of longhaired pigs who want a particular breeding but do not want to risk having the sow's coat chewed by the boar.
A sow who is constantly in heat may be exhibiting symptoms of uterine fibroids or cysts. She won't ever go out of heat, the uterine opening will stay visible day in and day out. A sow that has light bleeding in conjuction with her cycle may have uterine cysts, a condition similar to uterine cancer in women. The cysts should be removed as soon as possible to avoid the spread to other parts of the reproductive tract, but the chance of reoccurance is high.
To examine a sow and check her heat cycle, you must roll her back in your arm (I'm right handed, so I hold her in the crook of my left elbow) and use my left arm to hold her against my body. My right forefinger and thumb will be used to keep her little legs open so I can see clearly, and the forefingertip and thumb will be used to spread the labia so I can view her myetus (bladder opening), the status of the hymen (open and retracted, she's in heat, closed over the vaginal opening, she's not receptive), and the rectum.