If you have an adustable wrench, cinch it down on your bar (doesn't need to be supertight, just snug it up), slide the wrench off, measure the distance between the jaws with any old measuring device, convert as necessary to mm.
Measure where the bar is the same diameter as where the stem clamps the bar. Some bars taper smaller after the clamping section. While you're at it, measure and remember your seatpost diameter. Alot more variations there.