I went into the front pipe so I could be as sure as possible that the readings were unaffected by the cat. And at the same time I could check exhaust pressure wasnt too high. It is a quick easy test, I buy lambda sensor 'holes' so after the test I can just screw in a bung.
The only other clue was that when the manifold was off the valves were spotless and so were the crowns of the pistons. Everything looked washed clean to the point of looking virtually new.