If there's no length marking, and you don't have a string that fits (or is at least close) to use as a gauge...
Use a flexible tape measure (or a string, then measure the string). Start at the "top" of one string groove, follow the contour of the limb, through the center of the riser, follow the contour of the other limb, to the "top" of the other string groove. Subtract 3-4". That's going to be close with most bows.
A tillering string with a single loop on one end and tie a timber hitch on the other will work also.