What would be the reason why a car is overheating?
The cooling system is meant to be a sealed system under pressure. Each pound of pressure raises the boilling point by something like 5 degrees so if their are any leeks in the system that allow the pressure to escape that will lower the temp at which your car will over heat. Try replacing the rad cap. To test head gasket most garages have a kit they use to test for exhaust gases in the coolant but sometimes if it is bad enough you can tell by simply looking at the coolant in the radiator while the car is running. If it looks foamy after the t stat has opened chances are bad gasket. Other possibilities are poor coolant flow or air trapped in the system.
if you have anti freeze/coolant in your radiator, while the car is running put your hand under the tailpipe (exhaust) (dont touch the pipe, it will burn you ) just for a few seconds, if your hand smells sweet like the smell of antifreeze/coolant , its a good chance your head gasket is a gonner. another telltale sign is if you got steam coming out of your tailpipe all the time. also if you should happen to flush your radiator out and get oil mixed in with the coolant or water, that is a good indicator that your head gasket took a dump.
