First inspect the alternator, for loose wires, connections, etc. Then start the truck up and make sure belts are tight and nothing is slipping. If you can safely get to the alternator while the truck is running you could test it with a multimeter set to 20v. Ideally it should be putting out 13.8 - 14.2 volts. If you can't safely get to the alternator you can test at the batteries, but it's not as accurate.
You also need to load test the batteries, the proper way to load test is to first fully charge the batteries, then remove the surface charge by turning on the lights (high beam) for several minutes. Then attach the load tester and run the test.
You can buy both the multi meter and the load tester at any auto parts store and they are fairly inexpensive.
You may even get the auto parts store to test them for you. Most of them will do it on a car for free.