The most common sign and way to know if your head gasket is blown on your vehicles engine is to look for coolant and oil seeping between the engines head and engine block.
If oil and coolant is noticeable between the head and engine block then you have likely blow a head gasket.
Overheating is another sign of a blown head gasket but not always.
If you have a blown head gasket you need to get the head gasket replaced before the engine is ruined and you're stranded on the side of the road.