When the engine is fired up, watch how quickly the oil pressure builds. If the idle is high (the engine feels smooth instead of causing the whole truck to shake a little) it could require adjustment or there could be a hole in the diaphragm or the vaccum lines that go to it. A perforated diaphragm is usually accompanied by white exhaust smoke that continues even as the engine warms up. Both the diaphragm and lines are relatively cheap and simple to replace provided you buy Denso and not Toyota parts. You can test a diaphragm by pulling off the top end of the "clear" vacuum hose that goes to the rearmost nipple on the injection pump and blowing into it. If you can build pressure, the diaphragm is still good.
It is normal for a cold Diesel to blow a little white smoke at start-up. White smoke when the engine is cold can be caused by partially combusted Diesel. The smoke should quickly die down as the engine warms up. Blue smoke shows that oil heavier than diesel is burning. This can be caused by damaged piston rings or damaged valve guides and seals.
If the truck has a noticable miss as it warms up, there are three main potential problems. One or more of the glow plugs could shot. They should be replaced as a set. If all the glow plugs are working, a miss during warm up could either be a worn or out of spec injector or it could be due to lower compression in one cylinder.
On most pre-1988 Toyota Diesels, crank-case ventillation is achieved by a blow-by tube, which usually protrudes from the pushrod gallery cover on the side of the engine and extends down towards the bottom of the block. When the truck is running, check for excessive exhaust and oil mist coming out of the blow-by tube. Later Diesels have a hose running from the valve cover to the intake (upstream of the turbocharger on factory turbocharged models) if the intake plumbing and manifold are coated with oil, the engine probably has high blow-by.
The most common way to test the health of the valves and rings is a compression test. The compression test gauge threads into the glow plug hole of an indirect injection Toyota Diesel or into the injector hole of a direct injection engine. All glow plugs (indirect injection) or injectors (direct injection) should be removed before performing the test to allow maximum engine cranking speed. If one cylinder has a reading lower than the others, try removing the compression test fitting and pouring ~15mL of oil into the cylinder. If the reading improves when the compression is tested again, the problem is likely the rings. If the compression does not improve, the problem is likely either the valves, head gasket, or cylinder head.
Another common problem spot for pushrod equipped Diesel engines are the valve rocker arms. Because of the inherent tap of a Diesel's injection pump and the rattle of Diesel combustion, it is much more difficult to hear the ticking of poorly adjusted valves than with a gasoline engine. Many Diesel Land Cruiser owners do not follow Toyota's schedules for valve clearance inspection/adjustment. If the valves are run out of adjustment for an extended period of time, the valve stem will begin to beat its way into the face of the rocker arm. That increases the clearance and accelerates the problem. If left too long, a dimple will form in the face of the rocker arm. The motion of the arm then pushes laterally on the valve in addition to downwards. Once that happens, the valve will begin to rock and wear out the valve guides.