Here is a simple method, requiring nothing more than a ruler, a piece of tape and a flat surface: Put a strip of masking tape across the butt, about in the middle. Make a mark down the center of the tape, indicating the centerline of the butt. Lay the gun on it's left side, using a shim under the trigger guard, so the centerline mark will be parallel to the table. Measure from the table to the C/L. Record this measurement. Flip the gun over to it's right side and again measure table to C/L. Subtract the smaller measurement from the larger and divide by 2. This is the amount of cast-?. If the first measurement was larger(with the gun on its left side), the answer is cast-on. Good for right handers. If the measurement was larger laying on its right side, that is cast-on, preferred by lefties.
The two examples I measured were a Lefever:1-3/8" and 1-1/4". The difference divided by 2 = 1/16" cast-off. Hard to see, but there. The other was my .410 Uggie Falcon: 1-3/8" and 7/8". Difference divided by two = 1/4" cast-off. Easy to see and it fits me very well, that way.
