Rolla

Wham48 is correct, you might notice a white color smoke coming out of the tail-pipe if it's the head gasket (your cat may be hiding this though) to see if you have a heater core leak make sure you flip your interior heater on an switch the temp from cool to hot. run it a little while and on the passenger floor board you may find evidence of moisture. Last point, pull the spark plugs and read them to see if you have evidence of contamination. pull the oil dipstick and do the same, the oil will be milky white if the coolant has contaminated it.