There are a few squirrelly ways to determine rim ERD (Effective rim diameter)..here is one I stumbled across.
Step 1: Determine the length from the base of the screw slop of a nipple to the top of the rim. That may be a slightly tough measurement to get, but you can certainly fashion a tool. I like this method as it measures from a seated nipple, as opposed to other ERD methods that rely of measuring spokes as they come together at an imaginary center of the wheel. Those methods have variables in how much the nipple is threaded on the spoke to start.
Step 2: Get a cloth or other flexible tape and measure around the outside of the rim edge.
Step 3: Subtract twice the measurement from Step 1...that will give you the rim's ERD