Weights are good to have but they are really more useful if you have a record for the individual bird. A good vet does will weigh your bird but it is not really to see if they are over or underweight. They will handle your bird and feel the birds chest to determine the body condition.I dont weigh mine at all. Im sure the vet does when they go for checkups.
I am a cyclist, so I eat like a horse. Calories out = calories in. I use the same approach with my babies. Big flights, lots of playtime and branches to destroy all day
They get plenty of food, but only the good stuff. so far no vet has ever told me my birds are overweight.
( Though One did make a pass at Thumper)
A bird in condition will have a full chest but you will still be able to find the keel bone. If the keel bone is sharp, protruding or very pronounced it indicates that the bird is underweight. If the you can't find the keel bone under all that chest fat then, well, you guessed it your bird is fat