This is how you check it- or how an autospark will check it....
You need to check that nothings draining the battery when you aren't using the car: for this you'll need a multimeter, if you have one (they are only around a fiver from Maplin) set it to the amps range and connect it in series with the battery + terminal, there should be a only slight discharge for the clock,immobiliser, and radio memory, and you need to pull fuses one by one and see if the discharge changes: when it dies, you've identified a circuit (other than the ones mentioned above) that is draining current and that's where you need to check for a faulty component.