A good share of auto parts store can test it in the car two. I was usually satisfied with the alternator if I could remove the positive connection from the battery and the car stayed running.(That is if you are not overcharging, as this test wont tell you that) You can further load test it by adding the headlights and 4ways. Turning on as much as you can.(with the terminal off).
I would use a volt meter, a good alt. Puts out 13.8-14.2 volts. Also listen to the the alt. If it is working properly it has a steady wining noise, compare it to another car. If it makes a howling noise it has a bad diode or stator. If this does not cure your problem then you might take it to the parts store and let them test the complete system. Also with the engine off see if you can turn the alt pulley. If you can the belt is loose and it will not charge this way.
A real easy test but can be dangerous if you are not careful and pay attention to what you are doing. Take a screwdriver and hold it to the back of the alternator in the center while the engine is running, if it is charging it will pull the screwdriver towards the alternator like a magnet. An other way is to check it with a volt meter if you have one. Also if you want to take time to remove it, most autoparts stores will check it for free on a machine.
One way to check it is to start your chevy an take the positive side off battery if it stays running its prob ley good