Okay. String up the bow. Follow the part of the riser that tapers down and is sandwiched by the laminations all the way down to where it tapers completely away. That part of the riser is called the fadeout. Measure 90 degrees from the string to the end of the fadeout. Then do the same on the top end.

If the bow is a takedown, there will be fadeouts in the limb butts. Measure from the ends of those fadeouts the same way.

A bow that is tillered for split - most of them- will usually measure 1/8-1/4" closer on the bottom fadeout than on the top. If it's tillered for 3-under, it will measure even on each end or up to 1/8" closer on bottom.