Another thing to look for is combustion gasses in the coolant.

Go to a shop with an exhaust "sniffer".

Let engine cool.  Start engine up, get it warm enough for thermostat
to open (but not hot). Carefully open coolant expansion tank.  Hold
sniffer above exp. tank.  If it detects combustion gasses, you'll
need head gasket work.  :(

There is also a chemical way to check this, but I've never used a
chemical product.  I trust an electronic smog sniffer more.