With the engine cold, just put your hand close to the headers.
Have someone turn it on and rev it(or rev it urself with the cable), if the sound follows a pattern similar to how must gas ur giving it it is most likely the header gaskets.
Also, you would feel the heat and a bit of hot air from where the gasket is cracked or is missing a bolt.
DO NOT TRY TO DO THIS WITH A HOT ENGINE!!!! HEADERS GET REALLY HOT REALLY QUICK!!!