What's a good cheap device to check the charge an SLA holds?

I'm going to start testing the gradual degradation of these SLAs that I just got. However, I need the device that I can hook up to check what the voltage it holds. Don't know where to start or what it's called.

I went to Radio Shack and they were fairly clueless. They said they don't have anything that measures rechargeables like lawmower batteries and the like. I found an altimeter and they said it is for home electric current.

I also went to an autozone and they had simple battery chargers that had a green LED for fully charged and red for "in progress" I think there was another LED for "trouble". Anything that gives you more precise readings, perhaps in digital format?

Thanks.