Even easier is to leave the car for a few days (preferably during a cold spell). When you start it does it struggle and crank slowly? Also, look at the dash warning lights, if they dim appreciably during cranking then the battery is weak and ready for replacement. Occasionally batteries fail unpredictably but if you know what to look for with these 2 methods then there's a good chance you will not be caught out. Interestingly, the capacity of the battery can be substantially reduced due to age and it will still start the car, but you will be stranded if you leave an interior light on overnight. I left the sidelights on once by mistake for a few hours and that was enough. After I replaced the battery I tested it and it was only storing 4Ah, but it would still start the car!