Does your exhaust smell like coolant? or does your exhaust look white-ish? If so, that is a good indicator that it is a headgasket. Also, check your oil to see if it's a milky color?
Though, a very simple thing to do is check all your spark plugs and see which ones are cleaner than the others. The cleaner ones will indicate coolant. While you have your plugs out, do a compression test with a compression gauge (can be had at a parts store), it will go where the spark plugs go. Find some info on the compression your engine is supposed to have, then check it against your findings.