All you need is an accurate multimeter.

Bonnet up, everyting off, you should be hoping for 12.6 volts (12.2 abs min)

Now disconnect your injection system and crank the engine for 20 seconds, three times with 20 seconds between bursts. Get an assistant to do this unless you can read your voltmeter from the seat (or reach the key and read at the same time). As the end of the third cranking session approaches the battery should still be holding at least 10V. if not it's starting to fail.

now try charging (properly with a charger) and repeat to see if the results are any different (just in case th ebattery is healthy, but the drive cycles you use aren't keeping it charged).

Worth noting that no battery tester gives a 100% accurate result every time. And that batteries can and do die suddenly. but the above will give you a good indication of whether the battery is ok or not ok.

Once you've done the test and you've got your meter out you could do a basic charge system test whilst you're at it. reconnect your injectors and start the engine. let it warm a little and settle at normal idle. The voltage at the terminals should be at least 13, preferably nearer 14.5, but no more (everything off). Now switch on all the lights and electrical accessories you can. The voltage should still stay above 13. If it's less, raise the revs a little to see if it rises. If you get to 2000rpm and it's still under 13v you've got a problem.

Do these tests regularly (at least annually) and record the results and you may be able to detect a looming failure. i tend to do it during the Xmas shutdown when a) I'm home anyway and b) the ambient temperature is lower. Bear in mind though that the battery's ability to accept charge starts to fall below 5C and it won't accept any charge below about -2C.