When I order a bow from a custom shop I let them know how I check the draw weight. I do this so they will do it the same way because that's how I try to get what I'm after. I use the 1.75" towards the back of the bow from the pivot point. For most bows it is right at the back of the bow shelf. On my new Robertson, which has a more narrow riser, the 1.75" point is a bit further away from the back of the shelf.
I use a digital scale (Easton) to measure draw weight. I check every bow I have, no matter what the bow has written on it. Most are within a pound of what is written but I had one (Checkmate) that was 5 pounds heavier.
I wrap tape around a full length arrow so the near edge of the tape is 26" (my draw length)away from the bottom of the nock groove. I wrap the tape until it is raised about 1/4".
Then I put a piece of tape on the back of the bow's grip so that the top edge of the tape free floats in front of the bow shelf and just touches the underside of a nocked arrow shaft.
Then I hook the digital scale to the string and draw the arrow until the raised tape on the arrow contacts the tape in front of the shelf. That's my draw length and I read the scale. On bows where the 1.75" point is a bit away from the shelf I build the tape tab on the bow out a bit (usually this is less than 1/2 inch).
I also put a strip of raised tape on my draw length arrow at the 28" point so I can check the draw weight of the bow at this standard 28" draw. This tells me how much weight per inch my bow changes from 28-26".
The clothes pins or plastic clamps I've tried always fall off the arrow when I tried them instead of tape. Plus, I keep this taped arrow in my shop to use time after time.