If you immediately get strong cranking of the starter, then you know it's not that. So just take a screwdriver and touch both points together (make sure the handle isn't metal!). When the solenoid gets power from the ignition switch, it basically just bridges the connection and the 12v power turns the starter, kind of like an over glorified relay switch.
One connection will be constant 12v from battery, the other will be a wire from your ignition switch. Easy way to check the starter is to get under your truck and use a metal tool such as a screwdriver to bridge the gap between the two wire connections on the solenoid.
