First, just wipe them clean with a rag. If they're smooth, just put them back in. The outside of the cartridge bearing has nothing to do with the bearing's function, so there's no need to use any solvent to clean the outside. If they're gritty, either pry off the seals, clean and re-lube, or just buy new cartridges. It doesn't hurt to put some grease on the parts that touch when you re-assemble the headset, and maybe coat the seal with grease to bolster the seal. But all the moving parts for a cartridge bearing are on the inside adn cleaning or greasing the outside does nothing to improve performance. You can easily test performance by just moving the inner rotating part and feeling if it's smooth or not.