Heating systems are very simple compared to AC systems; after all, getting air hot is easy when you have a big thermal conversion engine not six inches away. Heater cores are the last link in your truck's air-heating strategy, responsible for transferring the heat of your engine to the air in your cab. Checking the heater core is easy once you know how everything works. Just bear in mind that heater hoses are, well, hot.
Determine whether or not your lack of heat is from a heater issue or because the coolant isn't getting hot enough. The simplest way to do this is to look at the temperature gauge, but you can guesstimate its heat by wrapping your hand around the upper radiator hose. If it's really, really hot, then your engine is heating. If your engine is running cold, then your thermostat is likely stuck open and the heater core isn't getting warm enough to heat the air.2
Turn your heater on high and locate your heater core valve. You'll find it in the middle of one of the hoses leading from your engine to the heater core. Gently wrap your hands around the hoses on either side of the valve. They should be the same temperature. If the engine-side hose is noticeably hotter than the core-side, then the valve isn't opening or isn't opening all the way.3
Wrap your hands around the heater core input and output hoses. The input hose is the one with the heater valve in it and the output hose is the other one coming out of your heater core. They should be the same temperature or nearly so; the output hose will typically run a bit cooler than the inlet, but it shouldn't be room temperature or cold after the heater valve opens. If it is, then your core is clogged.4
Go inside the truck and look for water in the carpet and floorboards of your truck. Smell the area around your dashboard and take a good whiff of the vented heater air. If you detect any odor of engine coolant, then you've got a coolant leak in the core and it has to go. If you can't replace the core right away, you're better off disconnecting the input and output lines and installing a piece of metal tubing into the line ends to loop them together. That beats ruining your interior with coolant.