A cheap digital voltmeter will tell you if the stator is charging the battery. With the ATV running, put the digital volt meter on the 12V DC setting and touch the test leads to the battery terminals (with the ATV's ground and power leads still attached of course). You should see the voltage jump up to roughly 13-13.5 volts. The voltage should also increase slightly as you rev the engine up. If you don't see an increase in voltage, you should then go on to checking the stator/regulator.
Find the plug coming out of the motor that typically has three fat yellow or white wires in a connector. Unplug it and check the wires for continuity to ground. If any of the wires are grounded... the stator is toast. If they test good... start by replacing the regulator!