after the car has been off for a few hours, pop the hood, and put a volt meter on the jump point inside the fuse box. should read 12.4 or better. 12.0-12.3 is iffy, less than 12 is bad. you can try charging it, and see how it holds, before replacing. a new, fully charged 12 volt reads around 13 volts. my 5 year old battery reads 12.56 volts.
