Threaded or threadless?

If it's the threaded kind measure the shank of the stem where it enters the fork. A 1" stem will actually measure 22.2mm because it has to be a little smaller than 1" to fit inside the tubeing.

If it's threadless, remove the top cap and measure the stem's inside diameter. As long as you keep the clamp bolts tight doing this won't affect your bike's headset preload.