How would I know if I had rheumatic heart disease? Had scarlet fever that went untreated longer than it should have 22 years ago.

An exam could help. Rheumatic heart disease is the result of a strep infection. Fortunately most don't go on to have heart involvement, particularly if the infection is treated. If your heart valves were affected it would very likely be audible on physical exam. If your physician is suspicious, they may order an echocardiogram to confirm those suspicions.
Echocardiogram. The definitive answer will come from seeing a cardiologist and having an echocardiogram done. That will settle the question once and for all. If you have no cardiac symptoms and no murmurs when your doctor listens to your heart then rheumatic heart disease is unlikely as well.
Should be obvious. The cited risk of RF after untreated strep(including scarlet fever) is about 1/200 cases. Symptoms usually appear within a month or so of the initial infection. If you haven't had any significant impairment to this point it is unlikely you had it. A thorough evaluation by your internist or cardiologist could confirm or exclude the issue if you are concerned.