First, looking at the the system. In a simple statement, coolant is circulated around the engine to pull heat away from the engine. When it reaches a certain temperature, the thermostat opens and the coolant flows into the radiator to help cool the fluid. The heater core in the dash allows warm coolant to flow into the passenger compartment to give you heat. The water pump creates the flow as it's spun by the drive belt. Regardless of how much modern technology goes into vehicles, not much has changed with this basic system. At this time, there is no tool that can simply diagnose the cooling system issues.
Diagnosing the system starts with a visual inspection. First the level of coolant needs to be determined. Low levels of fluid means that there is less fluid to carry the heat away from the engine, or into the passenger compartment. If the fluid is low, then add fluid start the car, and see what happens. If you don't see any leaks, check the exhaust. Smoke color from the exhaust can tell you a lot. Black smoke is a fuel or ignition issue, blue smoke is burning oil, and excess white smoke is water or coolant. This can be a little harder to determine in the winter time. Pulling the engine oil dipstick can also tell you if your car is burning coolant because the oil will look slightly milky.
So with the vehicle cold, start the car and turn on the heat. Wait and watch for the engine temp to rise. Periodically check the temp of the top and bottom radiator hoses. If one hose is significantly warmer than the other, you may have a restricted radiator flow issue or a water pump issue. Sit in the car with your hand in front of the vents. Press and hold the throttle pedal until the RPM's reach about 2500. If the temperature from the vents feels hotter with the RPM's up, it's more likely to be water pump related because it spins faster as the engine drive belt turns faster. So even a fading water pump works better at higher RPM's. If there is no change in temp from the vents, we go back to a restricted system.
Now because there is no guarantee of our diagnosis using these techniques, I often recommend the most basic check of all. A coolant flush. A good coolant flush machine can create a more powerful flow than the water pump and give the system a good cleaning. Most restrictions come from rust and scale. Once these are cleared out, flow usually is much better. Doing this in conjunction with a thermostat usually fixes most no heat issues.
****WARNING**** If your vehicle has over 100,000 miles on it and it's never been flushed, you may develop a leak that you did not have before. That's because the rust and scale that gets removed may allow for pinholes when it releases from the inside of metal components. It's not common, but it does happen from time to time.
Honestly, I haven't seen a radiator have to get replaced for any reason other than a leak in a long time. The heater core in the dash can also be directly flushed. If the heater core in the dash springs a leak, you'll know it. Usually there will be a puddle on the passenger floorboard and a strong smell of coolant. Replacing a heater core sucks! It is located behind the dash and requires major disassembly. This type of job will usually cost you the most. Also, if it's being done, be sure the shop is replacing the A/C evaporator core at the same time. It is located right next to the heater core and doesn't require any additional labor to remove. There is nothing worse than paying all the labor in the winter time for a heater core, then paying all over again in the summer time when the A/C won't work due to a leak in the evaporator. A little prevention is worth it's weight in gold.
Sometimes it may take doing a couple steps to get the system working right. I had a friend that had a Dodge Ram that we replaced all of the parts of the system and it still never got the right heat from the vents. Never did figure out why. He got rid of the truck, and I've never had a problem like that again. Fluke? I hope so.
So stay warm and drive safe! Remember, do your flushes long before you ever have a problem and you really will be less like to have these problems. I've seen it myself!