The criteria is similar to that of selecting a Remington Double Derringer.
Look at the hinge for evidence of old repair or cracking, then check the lock-up for snugness and the action for smoothness.
The weak spots are the lock-up and hinge.
Ideally, you can clean up roughness with a stone and good lubricant, but repairing the cracked hinge can be expensive.
'In general' - the exterior condition is a good indicator of overall care, so be prepared to spend a bit more for a nice-looking piece.
This will be a 'BP-Only' piece, and if carefully loaded, will shoot well - but don't get ahead of yourself and decide that a smokeless load might be safe.
Good Luck.
Vaya,
Scouts Out!