To test a battery properly, remove it from the car and charge it over night. Leave it to stand for a few hours then check the specific gravity of the 6 cells using a hydrometer. each cell should read around 1250 on the float or green depending what type of hydrometer you have. Then if all of them give good readings, you need to load test the battery by shorting it out with a drop tester. using the drop tester it should load the battery up for around 10 seconds with the voltage not dropping below around 10 volts during this time. Like wise, if any of the cells gas and bubble like a shook up bottle of lemonade then the battery is scrap.

Then you can always take it to be tested at a decent auto electricians.