Use a caliper or micrometer to measure the riser width thickness at it's widest several inches above and below the arrow shelf. Then measure the thickness at the center of the shelf, or wherever it's widest if it's crowned. Will need to take off the strike plate material for an accurate measurement, or subtract its thickness from your measurement.
Subtract that result/number from the average of the first two, if they're different. The amount that number is more or less than half the width of the riser is the amount it is cut proud of or past center.
Doesn't work if the arrow shelf area is off center, i.e., built out on one side of the bow as many metal ILF risers.