you could....go to all the trouble of getting everything tested.

or, just buy a battery and be done with it in 10 minutes. My money's on the battery. It's getting old anyway.

You can sort of use a piece of metal (like a leatherman or wrench) to check out the alternator. It should magnetize to the alternator while it's running. Just hold it near and see if it's pulled towards it. Obviously, don't let it get pulled into the belt. Not very scientific I know, but I'd trust it more than the clown working at your local auto parts store.

I've never actually tried that on the S, but I have on some other cars.

Good luck,

Jason