Start by pulling the plugs and checking for "wet" plugs which is a sign of unburnt fuel. Make sure to note which cylinder each plug is removed from. If only one plug is bad, you may have a bad plug, spark plug wire to that plug, a distributor cap problem, or a stuck injector. If more than one are bad, you may have a bad coil, distributor, or distributor cap & rotor. There are 2 coils and 2 distributors, so if it's coil or distributor related affecting several cylinders, you'll be able to trace the bad plugs to one coil or distributor.
Without any other information, my wild a$$ guess is your timing belt is slightly off causing improper firing (and excess fuel being sent out the exhaust).