Isn't there a simple way to find out? How about a couple of supecting miners decide to use a slightly modded miner that would inform the user when it finds a block.
It'd be sufficient for this group to be able to present only 1 block that was stolen by the operator.
Of course they would have keep their identities secret, otherwise the pool operator can just always leave their blocks untouched and choose blocks of other miners for stealing.
This would at least create a lot of danger for the pool operator. The pool operator getting caught stealing would certainly destroy the pool, so even a slight danger of that happening should keep him from stealing, right?
Am I overlooking something?
This has been possible from the beginning. Miners know if they have the winning share, and the pool reports who found a block. Unfortunately it's the only way I know of to truly audit a pool you suspect of withholding blocks, and it would require a large number of users to provide reasonable assurance one way or another.