I check a few of mine at the begining of the spring and again during the summer unless there is something obviously wrong with any of them, at which point I would re-check some of them. Also, when you net koi or a large fish, check the underside of them as they sit on the bottom of the pond during the cold months and they can get sores etc. on their bellies. I have a microscope and can take a scrapping off the fish to check for parasites and treat accordingly. By looking at your fish regulary you can soon spot if a fish is off colour, indicating that something is amiss. Keep a check on water quality as, if you think about it, fish live in a toilet of their own making!