How can you tell when boys are done growing taller?

Without radiography: As in x-rays to determine if growth plates have fused, you may not readily be able to determine if a gentleman will grow more. Remember some men have very late growth spurts.

Depends on bone age: The potential additional length anyone can expect depends on areas in the long bones (arms, legs, fingers)called growth plates. The active open growth plate means more growth is possible. An xray of the hand/wrist can be evaluated for bone maturity. If your bone maturity is =18yo you are done growing taller.If you've grown less than an inch in the past yr you are likely done.