If the battery is not a sealed battery you can use a hydrometer. The ones in local parts stores are small and difficult to read. I got a nice one from Amazon for $10 a couple years ago. You used to be able to buy these anywhere but now they seem to have those pencil sized ones. Bigger is better, I say.
The hydrometer will tell you if you have a dead or weak cell and the state of the other cells too.
If the battery is sealed you can't get a hydrometer in to check the specific gravity. At Harbor Freight I got a battery load tester for $22. http://www.harborfreight.com/100-amp...ter-61747.html
I've had it for about a year, check my batteries periodically. It's cheap but does the job. There have been some complaints about it wearing out in a short time but for $22 and over a year now, no complaints from me