Any cylinder that is not working as hard as the others is the most likely cause of emissions issues.
Most professional scan tools can accurately perform a cylinder balance test by dropping one cylinder at a time.
With an infrared thermometer, look for cylinders that are running hotter or colder than others. This can indicate a lean - or rich - condition. Likewise, it can indicate high - or low - cylinder compression.
Rear cylinders which are running progressively hotter than the front indicate cooling system deposits (rust or sludge) restricting flow to the rear of the engine block. This can result in high NOx emissions due to increased combustion temperatures, even though the average coolant temperature reads normal.