It's not hard to figure out what is wrong when an engine is overheating. Every time one comes to me, I follow this exact same process, and it has not done me wrong yet. People pay me a lot of money just for telling them this, and here it is for free.
You should start with a cold engine that has a completely full cooling system, that doesn't have any visible leaks. Some engines have a bleeder screw on the top of the cooling system, this is for a reason. When you fill the system, have the bleeder open and add coolant until it comes out the bleeder, then close it immediately. Most vehicles also require you to run the heater on high, while filling the system. This will get all the air out of the system.
OK, the system is full, and the cap is on it. Crank it up and let it run for about 30 seconds. Now squeeze the top radiator hose. Is it pressurized already? If so, bad news, there are exhaust gasses entering the cooling system, through a leaking head gasket or cracked head casting. A more expensive version of this test is called a 'Block Test'.
If there was no pressure on the hose, then that is definitely good news. Continue to let the engine run for about 4-5 minutes. Just feel the top radiator hose, it should warm up after a few minutes, as the engine warms up. It won't get that hot at first, but as long as you definitely notice that it gets hotter is all that counts. About this time, the (electric) fan should be coming on, too. If the hose doesn't get warm, replace the thermostat. If the fan doesn't come on, all the system is a switch, relay, and fan motor. You can figure it out from there. If not, click on the link, that says talk to a mechanic, and you might just get me!
Anyways, if the thermostat opened up, and the fans came on, then there are likely problems with the radiator or water pump. Water pumps are usually told off by a leak or grinding noise, when they go bad. To test a radiator, all you have tp do is let the engine completely warm up, and then shut it off. You need to feel the tanks on the radiator, not the fins on the core. Are the tanks different temperatures? If they are, then the radiator is clogged.
If not the radiator or water pump, either, then you should figure out why the ignition timing is too advanced, or replace the collapsing lower radiator hose.
Hope this helps, if these 'instructions' are followed completely, you will find out why your engine is overheating.
If you still need clarification on how these parts of the cooling system work, along with everything else between your bumpers, check out my profile, which I will be adding articles to daily.
Thanks for reading, and keep it cool!