Which CF6 engine is that? I have an AMM for a CF6-80C series engine on a B747-400 and a CF6-80E series on an A330.
The -400 mentions an optional test using a specific tester (Barfield TT1200A) for checking the probes and gives specific values for the probes.
The Airbus AMM only mentions using an ohmeter and looking for a positive movement on the meter.
I used to work on RB211 engines and the AMM specifies using a wheatsone bridge and gives specific values although we used to do a quick check with a fluke meter just to get a ballpark figure.
What kind of meter were you using? I never heard of swapping the leads and rechecking.
The main thing when doing the check with the proper equipment is that you need a really good meter connection because the voltage generated at the thermocouple junction is very small.