We questimate based on teeth, appearance and how they act, but we tend to just say 'adult' but on places like RR you have to give an age- so I just mostly make it up :lol:
(also, I have a lot of bunnies over 5 and all have nails the same as the younger ones (around 2-3) so I personally wouldn't take nails as a guidance but it can be a guindance as to whether a bunny has been neglected or not sometimes).