You will need a voltmeter. With the engine off the battery should read around 12.4 volts. With the vehicle started and nothing else on (lights, radio, ac, ect) should read right around 14.5 volts. If not then you have an issue. First check battery terminals. Check for loose cables, or corrosion, or frayed ends. Then check your grounds. Also check your alternator fuse. Also check all your connections at your alternator. After all those things you find no faults, then you probably have a bad alternator.
--- 1998 ZJ 5.9 Limited - 4" lift, 31 x 10.50 15, Jet chip, Kenwood Excelon everything, current DD
--- 1996 ZJ Laredo - Parts parts and more parts
--- 1993 ZJ Laredo - 4.0 Bone stock for now (New to me)