Best way i know is to draw an arrow on a bow youre really comfortable with to your anchor and have somebody mark where the arrow crosses the back of the bow.
If you have no friends to do this, take a big elastic and wrap in around the arrow, put the elastic 28" to start. Draw and hit anchor, if the elastic bumps the back of the bow before you hit anchor, move it further, if the elastic doesnt, move it in and repeat. Then, when the elastic is hitting the back of the bow right when you hit anchor, youve got your draw length. Thats the method I use. I found my draw was considerably shorter that i thought. 26" for a thick handle and 25" for a bendy.