One twin is balding faster than the other one.
The other twin with more hair, has a slightly stronger jawline, or slimmer face. I’ve never heard them speak or seen walk or see them talk with their hands. These often show differences. With many twins, one is seen as the leader or instigator, the other one seen as a possible follower, or someone willing to go along. This dynamic often goes all the way back to babyhood. The parents (usually) see it, if no one else does.
See post 13.
These twins pose all kinds of problems, sometimes just pranks, but could lead to a major constitutional crisis.