If the crimp is not sufficiently heavy, the recoil of the first few rounds will begin pulling the remaining bullets in the cylinder. To check the crimp load six rounds, fire one and check the remaining five to see if the bullets have begun to pull from the casing. Fire a second round and check again etc... If after firing the fifth round the sixth round has not begun to pull from the casing your crimp is good or too heavy.
If you want the lightest crimp possible then load six more with the lighter crimp and repeat the above sequence until you are satisfied.