The closest way to check it with dial calipers is....
Measure a cam journal and divide by two to find the centerline.
Then measure down from the cam journal to the base circle of the lobe.
Subtract this measurement from 1/2 the jounal diameter. This gives you the base circle radius.
Multiple that by 2 to get the base circle diameter.
Again, this is only acurate for +/- .002" or so.
Works well on a surface plate with V blocks and a digital indicator that reads in tenths (.0001)