Let it be...that's my suggestion. Other than that, unless you have someone on the inside (cop, skip tracer, SIL at the DMV) you won't have much luck.
Greg
Let it be.
If you're that concerned, keep a notepad in your car with the lic., date, and time that the incident occured. If you see the ass again, and he does something similiar, log it, file a complaint.