For the last 9 years I've been using a multimeter which I bought for £10 quid. I can check the battery voltage and the alternator, as rule of thumb a charged battery should hold between 12.6-12.9v. Then start the car and the voltage should be anything from 13.9v-14.4v which shows that the alternator charging system is working for my PDI. If your battery is below 12.6v its on its way out from my experience, you could try charing it over night (i use a Ring RCB320 workshop battery charger) and see if it brings it up to level, if it don't then get yourself a new battery. If your alternator is not getting you a minimum of 13.9v its on its way out so get that sorted before you flip the motor. Using the multimeter is cheap cost effective and gives me the confidence that the battery and charging system are good before its sold as like you I hate complaining whining customers post sale lol. If your up for forking out 100-200 quid you can get one which prints it out for you and it will save you some time in the testing part, personally i enjoy testing it with the multimeter lol cheap thrills i guess
Anyways i hope this helps mate.
Cheers, Amer.