1994 Lesabre....question about starting.

About a month and a half ago, my car died while I was out shopping. No interior lights went on, nada, it was dead in the water! Next day when I had it towed to the shop, I was sure it was a bad or failing alternator, but the battery was bad. Was only two years old, a high end Truck/SUV Diehard, and it wouldn't even take a charge when they tested it.
My simple question to all pros out there: is there a tester that I can purchase to hook up to my car that will diagnose whether it is a starter, battery, or alternator issue next time this happens to me. I can change either one out on my own without it costing me a tow and parts and labor. Thank you for anything you can suggest!

A battery tester will tell you if the battery is dead, or will test any battery for weak cells. And once running it will check gen voltage output, very common tool/tester.

Thank you for the info. I was told the same thing from someone else. He also mentioned that a certain voltage would tell me that the alternator was bad(if reading 12 or 14 volts I think he said.....).
Went on to say that if both of those are testing fine, then look towards the starter or ignition coils. I do have a spark tester for that.

Your battery may have died because of some condition on your car.