Here are six things to check out that would indicate a healthy betta fish.
Swims easily: Your betta should swim around naturally. If it looks like it's having a hard time then probably it's not doing well.
Shows good awareness: Does your betta respond to you outside of the tank? What about following your finger around or flaring up at it? A healthy betta fish keeps a close eye on its surroundings.
Has a healthy appetite: Bettas are eaters! They don't eat all of the time, but your fish should eat regularly and go after food.
Bright color: Whatever color your betta is should be clear. A betta that looks dull or drab isn't doing well.
Full, healthy looking fins: What does your betta's fins look like? They should be smooth and flowing. If they're torn, hanging, or have stuff on them then your betta is unhealthy.
A strong, healthy, smooth body: Your betta should be streamlined. If the scales appear ruffled or there's growths and sores on the body then treatment may be necessary.
Don't just leave your betta in a corner and feed it once in a while. Regularly inspect your betta to make sure that it's doing well. Your fish will be a lot happier and live a lot longer.