Hello guys,

My 2005 Chevy Aveo started to have white smoke under the hood when I was going home one month ago. I had it towed to a mechanic who said it was the thermostat housing that broke. 3 weeks later, I am experiencing engine overheating issues. I took it to the dealership, and they diagnosed a "potential" blown or cracked head gasket but they are NOT sure… Since they tried to sell me a new car right away, I am not sure they are trustworthy.
I thought that the head gasket issue was easy to diagnose??? Does anyone could give me advices to check if it's really a head gasket issue???
My first thought was the cooling system. How can I check it?
A little history of my car: I bought it in 2009 at 67,000 miles, and never had any problem until this thermostat housing issue. I changed the timing belt and water pump at 90,000 miles, and valve cover gasket, spark plugs, air filter this month along with the thermostat housing (metal, not plastic).
Even if the head gasket work is expensive, I feel like it's worth it to get it done since my car had no problem before but I would love Aveo experts opinions!
All your comments and suggestions are be much appreciated!
Happy Holidays!