Yank it out, measure the internal and external diameters, squint at the upper and lower angles, look for appropriate bearings in stock of 9 different types of cartridges, realise you don't have THAT one, scream, throw bearing against wall etc etc
Something quite helpful (more for the pros....) is the FSA profiled steel tool which helps you with angles etc- sometimes the seal colour tells you which bearing you have if the markings are rusted off etc
For cartridge bearing headsets:
Yank it out, measure the internal and external diameters, squint at the upper and lower angles, look for appropriate bearings in stock of 9 different types of cartridges, realise you don't have THAT one, scream, throw bearing against wall etc etc
Something quite helpful (more for the pros....) is the FSA profiled steel tool which helps you with angles etc- sometimes the seal colour tells you which bearing you have if the markings are rusted off etc