Just check the resting voltage on the battery overnight.. First top it up with electrolite or a bit of soft tap water or de-ionised water, and then give it a charge for a few hours. Then leave overnight and check the voltage. No point buying a new one when you don't have to..
I'm all about saving the money
I have had some problems with batteries causing all sorts of weird issues at idle, but alas, One of my last batteries used to get pretty low after long time of non-use. At the time, I could start the bike but i'd have to start riding away for half a mile or so before I could turn lights on or use any indicators before the charge in the battery + alternator output was happy.
Anyway, as mentioned- check resting voltages, they are the best idea in regards to battery life.