I jack up one wheel at a time starting with the right front, and I shake the tire top in and out to find any free play in the kingpin or bearing," says Mike Beckett, one of the founders of wheel-alignment-equipment manufacturer, M.D. Alignment.
You'd need a dial indicator to measure the .01 inch or .02 inch of end play that most axle manufacturers say is acceptable tolerance for truck and trailer wheel ends. Most mechanics can detect in-and-out movement by giving the wheel assembly a wiggle. It's not precise measurement by any means, but experienced hands can usually tell if there is too much play.
Put a spotter in position to watch the brake canister, and if it moves with the wheel, it's a good bet the kingpin is worn. If the wheel moves by itself, the problem is probably confined to the bearing.
Beckett recommends checking front-to-back movement similarly, by grabbing the tire at the 9 o'clock and 3 o'clock positions and shaking it side to side. Any movement suggests that a tie-rod end or a drag-link ball joint may be loose.
"Two points of caution should be mentioned," Beckett adds. "One, don't grease the truck before attempting this. The grease will fill the available spaces in the kingpin assembly and you won't find the movement. Second, if it's real cold out, 20 degrees or less, the grease can be so thick that you won't find movement."