That's a good question. I'm not sure if there's any blood test, (at least none that I'm aware of), to diagnose RLS in particular. But maybe some of the tests for various enzymes can give clues? I'm not sure.
If you have RLS, an excellent "remedy" that I have learned, used, and passed successfully along to my MIL who does this daily,....is Quinine Water. One glass before bed (straight, or mix with a juice). They also make an OTC product called Q-vel that has Quinine in it. Not sure if it's as effective or more so.
Want to emphasize, I'm not a doctor, so check with your doc and make sure he/she says it's okay for you to consume.
Let us know how you make out!
M.