My experience has been that a battery should be about 12.5 VDC with key off or when idling. When you raise engine speed to 4K or so, should be 14.5 VDC.
To check the electrical system, best way is to first get bike started and disconnect the three wires from the stator and measure AC voltage across them in pairs; that is A&B, A&C and then B&C. A shop manual will give you an expected value but it is like 40 VAC per leg and all legs should be about the same.
Then check your regulator/rectifier by measuring voltage at the battery at idle and 4K rpm. As noted, if it is working fine, should be nominally 12VDC at idle and 14.5VDC at 4.5 RPM.... if it is less, battery won't charge; if more battery will boil (over 16 VDC at speed).
Then you can check your batter with a hydrometer if it is a wet cell battery. They sell small ones at any autoparts store and they need to be small to accommodate the small hose you put on the end and stick in the cell... You draw out a little fluid and there are colored balls in the clear part of the hydrometer. These tell you the condition of the battery and if all float, the cell is healthy and is some or none float, it isn't. If you find a bad cell, get a new battery. If you have a maintenance free battery, any autoparts store will be happy to load test it and tell you if it is healthy.
A good preventive maintenance tool is a voltage gauge which can be hooked to the battery or a hot wire on the bike. KuryAkyn makes a nice little gauge that uses LEDs to tell you what voltage your bike is making and is a great way to monitor charging system health on the fly.
