What is the truck? Chevy or GMC? What year?
There are lots of things that might be clues. If it has any lights check the bulbs to see if they are six or 12 volt. That's not conclusive, but its a start.
If it has a coil look for a voltage indication on the coil. You can also check the starter, generator, regulator, horn relay and other items like that to see if any are marked 12 volt.
Does it have a radio? The radio might supply more clues.
If you are trying to restore the truck, knowing what it is will get to the answer. If instead you are just trying to make it driveable, it might be easier to determine what the parts you have support, and then finish that system.
