With the engine COMPLETELY cold, remove the rad cap and check the coolant level. If you can' actually see any coolant in the rad, there's probably a leak. If a B/H series Diesel is overheated or run with inadequate coolant, the heads are prone to cracking. Carefully look at the rad. Usually leaks will show up as whitish stains. When you return from a test drive, mist the rad with water. If all the water evaporates then the tubes aren't plugged. If there are areas where the water evaporates and others where it doesn't [cold spots, usually vertical sections of the rad] then the rad needs to be serviced [power flushed or rodded-out]. You can also check for cold spots by using your hand but there is often not a lot of space between the grille and the rad--careful not to burn your hand/arm.
Check all the radiator and heater hoses for excessive swelling. Also check at the end of each hose for white, pink (red coolant), or light green (green coolant) build-up between the hose and hose-barb. The build-up will indicate that the may weep at elevated pressure, either because of corrosion of the hose barb or because of age cracks in the rubber.
An easy way to test the health of the cooling system is to do a pressure test. A special pressure test pump/gauge that substitutes for the radiator cap and allows pressurizing a cold cooling system to the maximum level allowed by the radiator cap. While the system is under pressure, all the hose to hosebarb joints can be inspected for weeping, as can the radiator. If the pressure in the system bleeds down rapidly, there's a leak somewhere. Most pressure testers also have provisions for testing the opening pressure of the radiator cap.
A leaking heater core will appear first as a sickening sweet smell in the cab of the truck. In the 4x and 55 Series, the heater core is relatively accessible. A serious leak will result in a small puddle of coolant under the heater unit. The heater core in later models is usually much harder to inspect and harder still to replace.
For models after the 40 Series equipped with a rear heater, the metal heater lines run under the body and can become corroded and leak.