Car batteries are fairly low-tech items that many car owners forget about until they go bad. Batteries have a limited lifespan and knowing how old your battery is will help you to know when you need to replace it.
Open the hood. The car battery is located under the hood, usually in the front driver-side corner. Most cars have an internal release lever located near the driver's left knee that you pull in order to disengage the hood. If you are unable to locate the lever, consult your owner's manual.
Prop the hood open. Once the hood has been released, open the hood by unlatching the front. There will most likely be a thin metal rod across the front to prop up the hood while you are working. Be sure the hood is secured.
Locate the battery, most likely to your right if you are standing in front of the car. The battery will be stamped with a coded date. The code starts with a letter and a number. A represents January, B represents February and so on. The date will be one number representing the year of manufacture. The year 2008 would be represented by an 8 for example. If your battery is stamped C7 it was manufactured in March of 2007.
If your battery is too dirty to read the date, carefully wipe with a damp paper towel. Check the date code on a battery you are buying new as well. The date represents the manufacture of the battery. Don't buy a battery that has been losing charge on the store shelf for an extended period of time.
If the top of the battery or terminals (the knobs on the top that connects the battery to the car) appear flaky or white, the battery may be leaking acid. Put on gloves before proceeding.