Fork Stem Diameter
First thing is to locate the fork stem, as on some motorcycles there will be a plastic cap covering the fork stem, normally this can be removed to allow access.
Once you have access to the fork stem you will need to measure the internal diameter of the fork stem. Please also check that the fork stem does not get narrower further down, it's not very often but on some bikes this is the case.
Simply measure the internal diameter of the hole as the fork stem attachment will expand out to grip the sides.