It is very difficult to tell, as a lot of bunnies seemed to have found the fountain of youth to the untrained eye.
With credit to Dana Krempels, here are a few signs to look for that *may* indicate a more mature rabbit.
1. rough, opaque claws
2. greenish pupils (sign of lens thickening with age)
3. brownish teeth
4. stiff gait (possibly from arthritis)
Bunnies can live into their teens with proper care, so don't worry if you're bunny isn't a "spring chicken", he or she can still enjoy plenty of years with you!