A very efficient way of testing a catalytic converter for efficiency, especially with a failure of emissions, specifically a NOX (Oxides of Nitrogen failure).
Ensure engine is at operating temperature and utilize a digital thermometer.
A fairly inexpensive digital thermometer: https://www.amazon.com/EC-Technology-Infrared-Temperature-Thermometer/dp/B01HXNUVQE/ref=sr_1_1?ie=UTF8&qid=1471888807&sr=8-1-spons&keywords=digital+thermometer&psc=1
The key is a difference of at least 100 degrees from the inlet and outlet. This typically indicates the catalyst is not taking place and can cause higher NOX readings.
Ensuring that the air-fuel management system is dialed in is crucial, as in most occasions these catalytic converters will not fail on their own. Make sure that the air-fuel management system is working as designed. (Spark plugs, ignition wires, mass air flow sensor, fuel injectors …to mention a few ).
Additionally, as it is highly the case here, if your engine is consuming oil via oil rings or valve train, this oil will eventually embed itself in the substrate of the converter, and eventually, cause damage.