How can they tell if CM's R being used?

[Matty]

Without a confession, how can an examiner know with certainty that CM's are being used, unless of course they have sensor pads that indicate it but minus that, how can they know what one is thinking?

[Sergeant1107]

how can they know what one is thinking?

That is an excellent question.  
 
Since knowing what the subject is thinking would seem to be a vital factor in determining if he or she was using countermeasures, it would appear to be impossible to positively determine.  They likely make "educated" guesses and later call them "science."

[Bill Crider]

my guess is if they see an outlandish change in patterns they suspect CMs or if they personally witness the subject doing something odd in the chair.

[Dean_O.]

I'd be shocked if any polygrapher was able to detect a candidate employing "mental arithmatic" while answering a control question.