How to tell if my van needs a headgasket replace?

I have a 1997 Ford Areostar and repaired a heater hose and change out the thermostat. Once the thermostat kicks in the coolant is being force out the overflow. I was told this is either requires a new head gasket or purging the air out of the system. The van does not smoke and accelerate good. How can do I force out the air? or determine if it needs a new head gasket.