I've been having overheating troubles recently. I changed the thermostat and right after noticed that my radiator was loosing antifreeze. There are no visible leaks or drips, no signs at all of antifreeze dripping out. My dad suggested starting and running the vehicle with the rad cap off to look for movement of the fluid to show if the new thermostat is opening and antifreeze flowing. He said to also keep an eye open for bubbling when I first start it up, as that may be a sign that the head gasket is gone. Well, I just started it up and right away there is bubbling and the antifreeze wants to force out of the radiator. Is my head gasket gone?

It sounds like it, but if you want to be sure, you can get a combustion gas tester. It runs about 25 or 30 dollars and it's a tube you fill with a blue liquid and place over the radiator opening while the engine is running. If the gases are from combustion, the fluid will turn yellow.

The last thing I want to do is a head gasket.

How do I find out what side the leak is on? Compresion test?

Is there any chance that it's something else. A stuck thermostat wouldn't cause this kind of back pressure, would it? The top Rad hose is warm after warm up so I'm assuming the Thermostat is working. What about a clogged radiator? I know I'm grabbing at straws but I don't want to believe it's the head gasket.

You had a good idea with the compression test, I would start with that. If the bubbling occurs at start up with the engine cold, something is displacing a large amount of coolant....unfortunately, its probably exhaust gases.