[HO455]

Double check the easy stuff first. Battery cables and connections. Both positive and the ground leads to the block. It could be as simple as a loose battery post clamp or a broken ground strap. There are three wear components in the starting system (4 if you have an automatic) The ignition switch, the solenoid and the brushes in the starter. On our trucks the switch is the easiest to test as it is easy to remove and check with a meter. Or duck under the dash and test it installed. Test from the start terminal to ground with the test light or meter for the installed test for voltage. Or if you have removed the switch, test for continuity between the BAT terminal and the ST terminal while turning the key to the start position. If that checks out use your tester of choice to see if you have voltage to the solenoid when you turn the switch. If you have voltage and it doesn't turn the starter then it is a toss up between the solenoid or the starter. At this point its going to be time to remove the starter. I would end up tearing the solenoid and starter apart as I would just have to know where the problem was. But most folks would just replace the whole unit. As your checking things out be on the look out for melted components like the ignition switch plug I removed from my Burban. With all these tests repeat them ten or so times each since you're searching for an intermittent problem.
Good Luck.