[Flieger]

How much resolution do you want? You could setup a fixture for the bow with a pulley that has a string on it with one end attached to the nock or whatever the place is that you pull the bowstring. At the other end of the puley string you have a hook or some other method of attaching a weight. Then you can use some nice calibrated weights and measure the displacement of the bowstring with calipers or a micrometer or a ruler or whatever.

But if you want more data points, or if the force you exert goes down as you pull it like going over center then that method won't work.

What about a digital fish scale? Or do you want something you can log data in Excel?