Not sure but I think an exhaust shop can use a sniffer. I had one break internally on me and a similar thing happened to me. Then, it started to get worse. After a long drive, the engine started to "clog up" and before I knew it I could not go more than 25 mph. I thought it was starving for fuel so I replaced the fuel pump and still had the same problem. Had someone mention that it could be the catalytic to me. Took it to an exhaust shop and at first they said that wasn't the problem. I persisted and low and behold, that was the problem.