a faily easy way to test it under load is to turn on the ignition so it is in the "RUN" position ie as it would be when driving. then put a DVM accross the battery you should be detting about 13.2 volts with every thing turned off (there will be a small current draw ie pumps priming, ecu etc, then switch on say the rear heated screen, this draws quite a lot of current ! and see what the DVM now reads if it drops noticably then you have a old tired battery. if it doesn't you are fine!

hope this helps