Easiest way is to simply draw an arrow and have someone else mark the arrow with a felt tip pen at the back of the riser while you're at full draw. Do this several times and take the average length.
To do it by yourself, you can use a clothes pin that opens by squeezing the two long ends. Put one about two-three inches from the end of your arrow, then draw the arrow on the bow. When the clothespin hits the back of the riser, it should slide aloong the arrow toward the point. Measure the distance from the nock groove to the clothes pin. Again, do this several times to get an average reading.
Good luck.