You need a DC Voltmeter. Measure the voltage across the battery without engine running. It should be right at 12 volts within a couple of tenths. Then start the car even if you have to jump it. The voltage across the battery with engine running should be about 13.8. If it is less than 13 then use your voltmeter for a voltage drop test. Measure from negative battery post to alternator case and it should be less than .1V or so. Then measure from the output connector on the alternator to the positive battery post and it should be less than about a tenth. If it is more on either of these you have a bad connection, find it and fix it.

If the voltage drop is okay but the voltage across the battery with engine running is bad, then kill the engine and remove the voltage regulator on the back of the alternator and inspect the brushes. If worn to a nub or near a nub, click on fastlane above and buy one of their $15 voltage regulators and that will in all likelihood fix it.

BTW, a search would have found dozens of posts like this with the same instructions which would have prevented me from keying them in again.

Good luck,