I remember watching a machinist pulling his hair out while doing a very long facing job on the lathe, truing up a 2ft diameter lapping plate. The surface finish went to pot every so often across the cut, and he just couldn't fathom it out. This was on a large Triumph lathe.
You could have timed his smoke breaks by the surface finish. Every so often, he would park his a**e against the lathe drip tray and roll himself a smoke, and light it up. Once he went back to check the cut, it was cutting perfectly, but the previous few minutes it had been cutting crap.
As John said, forget about checking, just don't lean on the machine while it is in action.
Bogs