I'm relatively new myself, but in the past I've used the eyeball test. If the fish looked thick throughout and the belly was full I assumed healthy and thriving. On the flip side, if the fish had a big head and small body structure I would assume malnourished or old/weak.
I've since found a really good app - It's called SmartFish US
It uses length and weight to measure relative weight and its completely free. There is a paid version too if you want more detail / tracking ability.
I will say, the old eyeball test still works just fine; I've just got a name for it now in relative weight
