Check for white exhaust smoke
Inspect coolant level for unexplained loss
Look for engine overheating
Check for milky oil on dipstick or oil cap
Inspect coolant for oil contamination
Look for bubbling in the radiator or coolant reservoir
Perform a compression test
Perform a leak-down test
Use a block test for exhaust gases in the coolant
Check for rough idle or misfiring
Inspect spark plugs for coolant fouling
Watch for coolant pushing out of the overflow tank
Check for hard upper radiator hoses when cold start occurs
Verify no external coolant leaks are present