About the only test, you can do is in the bike, with a fully charged battery, and the spark plug leads removed from the spark plugs, and a spare set of plugs installed in the leads and grounded to the cylinder fins.
Put a multimeter on the battery terminals, and after 30-60 seconds of continual cranking, the battery voltage should not be below 10.5-10.6 volts.
When you dis-assembled and cleaned your starter, did you re-oil the sintered bearings at each end of the armature shaft ?
If anyone is wondering what a 'sintered' bearing is, its a plain friction type bearing, that is made from compressing a powdered or granulated metal into a cylindrical shape, it has a lot of nooks and crannies, that can 'absorb' or hold oil, and it seeps out over time to lubricate the bearing surface.