It is not the best means of testing, but it gives an idea.
Look for 12.6+ volts at the battery with the car off. With the car idling, look for ~14 volts at the alt and battery (if there is more than a couple hundred mA drop from the alt to the solenoid or battery, I would replace that cable(s).
A parts store can test the charging system in the car or they can bench test the battery and alt if you bring them in.