How do I test for a bad regulator or stator on a 2004 gsx-r?

[The Voice of Reason]

What causes these things to go bad?

[Mick]

check voltage across battery terminals @ 5k RPM... should be around 14.2v, much less bad stator, much more bad reg/rec

You have to have a good battery to test this though.

[guardrailjim]

Top Contributor gave you the procedure to check if the charging system is working.
And yes you do need a good battery for this test to be accurate.

Unplug the stator connector (the stator is in the left crankcase cover).
Set your multi-meter to 100 AC volts.
I'm not sure what color wires Suzuki uses, they used to be yellow and probably still are.
3 of the stator wires should be the same color.
Label each wire as A,B & C
Test the voltage output of each winding (pair of wires).
Put you meter's leads on A&B then B&C then A&C
Start and run the bike.
Rev the engine and the reading of each pair of wires should go well above 50AC volts.
If yes - the stator is good - the problem could be regulator/rectifier, bad wires/connections, or dead battery won't take a charge.
If no - there are a few more tests to check if the stator needs to be replaced.