NAPA has a "block check" kit that includes fluid that you draw the vapors from the radiator coolant through to check for combustion gasses in the coolant. I think the one we bought last week was about $50.00 for the kit. A definitive test.
Other than what's already been mentioned . . . Check and adjust the coolant level. Run the engine until it's good and hot (even drive it instead of letting it idle), and the temperature has stabilized for about 15 minutes or so (in a place that you can let it run that long and not kill somebody/something!), then make sure the radiator is pressured up as it would normally be. Then, turn the engine off and let it sit for about a day. The next step (after everything's cooled down) would be to check the dipstick for condensate or milkiness in the oil color. Then check the coolant level in the radiator to see if it's where it was previously. Finally, get out the spark plug sockets and such and take the plugs out of their respective cylinders. They should all be dry of condensate and have the same insulator coloration (or at least pretty close). Oil-fouled plugs would have the typically oily deposits on their insulators. If any one
or pair of adjacent cylinders are different, that might be where things are not what they need to be. If you don't mind making a potential mess, with the plugs out you could spin the motor over and see if any "foreign" liquid comes out of the spark plug holes.
This procedure might take the longest to do, but would be the least expensive (a new set of plugs?). Using the block check fluid would be quicker, but more expensive.
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