how can I tell if the car has a blown head gasket

There is a few things you can do first without tools, look for signs of coolent leak in the oil. It will show up as white milky coating on the dip stick, inside the oil filler cap, or oil in coolent of flow tank, and in radiator. If that all looks good, open the radiator by removing the cap and left engine idle to normal operating temp. If you see bubbles coming out of the opening you might have exhaust gases leaking into the cooling system. Next you will need to do a cumbustion test, and or a compresstion test, leak down test to verify the problem. These reqiure special test to perform and my be better to have a shop do it for you.