Vehicle batteries are fully charged at 12.7 volts. They can read a little higher if they have been recently charged.
The first step on a maintenance free battery is checking the eye on the battery to see if it is green. If it is clear or black, your battery is not healthy.
The second step is to check it's voltage. You can do this with a voltmeter or a multi-meter. Put the red lead to the positive post and the black lead to the negative post on the battery. (On most multi-meters turn the dial two to the left from the noon position for the correct range.) If it displays below 12.5 your battery needs a charge at the least.
The third step is to apply a load test. If you don't have a load tester, you can still use your voltmeter or multi-meter. Just leave the leads on the battery and have someone else try to start the vehicle. Look at the display and see if it drops below 9 volts while starting. If it does your battery is bad.
If you can jump start the car a few things are possible but in most cases it means the battery isn't healthy.
To check if your alternator is bad, look at the display on the volt or multi-meter with the leads touching the battery posts. It should read around 14.4 volts with the vehicle running. If it stays down in the 12 volt range while the vehicle is running, it means the alternator is not putting out power to recharge the battery. In this scenario, the battery has been trying to keep up with demand while not getting recharged. This is distinctly unhealthy for the battery and in a lot of cases in can ruin a perfectly good battery.
No tools at all? Check to see if the headlights work and are bright. If they do and are, listen to the sound if any the car makes while trying to start. Is it clicking? That is usually the starter. It could also be a loose connection to the battery posts.
Totally silent when you turn the key? With dim or no lights? That suggests a stone cold dead battery.
Is the engine steadily turning over but not firing? That's not the battery.
Is it a making a whir....whir...whir....sound and lugging but not starting? And a jump start works right away? That is likely a bad battery.
We test all batteries for free in or out of the vehicle, and will always answer any questions.