When the engine is up to full operating temperature and the tstat is open, you should be able to hold the upper radiator hose lightly but not be able to hold it tight. If you can't hold it lightly, then it is likely overheating. If you can hold it lightly, it is safe to drive.
Check the heater hoses. If it is hot returning to the block with the heater on full, then you are getting flow through the heater core and the water pump is working. And also, there is no air in the system.
If you want to check the head gasket, open the radiator cap and let it come up to temperature. If the head gasket is bad you will get bubbling in the radiator.
Hands and eyes are still one of the best guages.
If you want to check the tstat, just put it in a pan of water on the stove and watch it heat up. As the water just about comes to a boil the tstat should start to open. If the water boils and it doesn't open, then it is bad.
If it opens too soon and it takes too long to boil the water after the tstat opens it is opening too soon.