Pull the battery and take it into a battery shop... they can test it to see if it is still viable. If the battery has been left in the bike over winters and frozen and not been on a trickle charger it can fail in a matter of a couple of winters depending on temps.
I rencently 'bought' (quotes explained in another thread) a great battery charger at CT... 2-12-75 volt settings for trickle charge , full charge and jump start... handy to have with 2 cars, 3 bikes and a boat battery to worry about ;D