You could perform a head gasket leak test. You could get the computer scanned for both soft and hard codes. What's the condition of the coolant? Does the heater blow hot air? Does the coolant appear to be circulating?

Might be:
Blown head gasket (but you should see white smoke out the tailpipe if it is)
Thermostat stuck closed
Water pump not functioning
Cooling fan(s) not activating
Cooling fan switch not operating
Fan relay bad
Engine temp sensor bad
Radiator clogged