Using a Volt meter on DC Volts check at the battery post with the engine running. You should have at least 13.5 volts just off idle up to a max of 14.75 volts at higher rpm. If you do not see this follow the steps below. If you do then need to check with an amp meter hooked up inline between the positive battery cable and battery. Turn the key on and check to see if the amp meter reads negative. If it doesn't flip your leads. Now check to make sure you see a positive number when the engine is revved up over idle. If not, follow the steps below. If both these checks are good then the charging system is working and you may have a suspect battery.
Step 1 - Locate the three wires (usually yellow or white) coming from the stator in the engine. You will find a connector on this wire. Disconnect this and check for resistance between each pair of wires. (3 checks). You should have a approximately 1 ohm resistance.
Pass - 1 ohm on each pair and no continuity to ground.
Fail - High resistance or continuity to ground.
Step 2 - Next check for AC voltage between each pair of wires as above with the engine running at approx 3,500 rpm.
Pass - You have over 50 volts between each pair and the 3 checks are within a couple of volts.
Fail - You have low voltage or the voltage varies between each pair.
If you pass these tests then the stator is good. You then need to move on to checking the Regulator/Rectifier or possible damaged or corroded connections or wiring.