Well, the easiest way (though not as accurate) would be to take any full length shaft and put a tape measure next to it. Make sure to place the end of the tape measure at the bottom of the nock string groove and then mark in inch graduations up to 30-31 inches. Your arrow should look like it has black lines placed across the shaft at equal intervals.

Then, take a ruler or better yet a bow square and place it against the grip of the bow so that one edge is "even" (from a side view) with the deepest part of the bow's grip. Note where the straight edge passes up over the arrow shelf and mark that spot with a bit of erasable marker. That mark has to be easily seen though so don't go with a something you won't be able to see at full draw.

Then just draw the bow back with the graduated arrow nocked and see which graduation on the arrow passes over the mark that you made on the riser shelf. If it is in between marks then you can assume it is a "28.5" instead of a 28 or 29. After you get that number you need to add 1.75 inches. That will give you an approximate idea of the draw length of the bow....assuming you are not overdrawing it when at full draw....and, ofcourse, an arrow that is made to measure the draw length is much more accurate but beggars can't be choosers....