I'm not (yet) a car guy, but I try to be an educated consumer. Last year, I went to start my car and it had a dead battery. I figured I'd left the dome light on or something, so I jumped it and off I went. Get to destination, car sits a few days, it's dead again. Take it to AutoZone, assuming the battery is shot... turns out the battery is fine but the alternator tested bad.
I get home, break out youtube and ye olde multimeter and sure enough, one of the diodes is bad, causing a parasitic drain. I briefly considered swapping the alternator and trying to replace the failed diode on my own, but my don't-mess-with-it spidey sense tingled, so I called up the dealer and had the parts folks order an alternator.
I showed up a few days later and the service advisor explained to me that I shouldn't have been able to order the alternator because they didn't do the diagnostic (which is understandable from his point of view), but ultimately relented and did the work.
Similar experience in the summer. On a hot day, it sure felt like my (hydraulic, as it turns out) clutch needed to be bled. "We couldn't reproduce this, therefore we cannot do the work".
So yeah, I have jack stands now...