Right, pulling this out of the archives(from before I converted to the Arrow and spent money even quicker!!).......
Lean the engine by going to max egt then back by one division or use the above rough running procedure to acheive what you perceive to be the correct mixture setting. Now apply Carb Heat. This shold ensure that you have not gone too lean (which leads eventually to PA28 Gliders) because:-
Too lean a mixture will give NO RPM drop
The correct mixture or richer WILL give a RPM drop
So this can be used even without a CHT gauge. The trick is to set the RPM accurately so that the beginning of the reduction of the RPM drop with Carb Heat is noticable. It is VERY important to remember to go FULLY rich before any change in power setting or climb/decent is made and that FREDA chacks are made properly with special attention given to oil pressure as well as temperature.
However I do not remember the reason why this carb test works(first channel crossing to Alderney with the Club CFI right seat). If anyone thinks they know I would love to hear.