A little tricky.
1. You must have a known standard cam. Rotate it to the heel position and using a lifter and dial indicator -measure your inital base number. Slide the lifter out while leaving the dial in place.
2. Install suspect cam at the heel position. Install lifter under the dial indicator and check the difference.
For SBC's the usual standard base circles are .950, .900 or .875. A custom cam may have any possible number.
O