Equivalently stated, how do you know your reward is diminishing because another pool is block withholding your pool (results in share dilution in the same way as pool “share dilution”)?

The answer is the same: you don’t. You just receive less money. However it’s in the pool’s best interest to reward miners the maximum they can or else miners leave to competitor pools with higher payouts. Riot left Braiins after a period of bad variance luck, and that could be because entropy, or potentially a little competitive block withholding. Thats the motivation for competitor pools to block withhold: ruin payouts in one pool so miners get a bad impression and switch pools.

Mining pools have a principal-agent trust dynamic that is inherent to the distribution of work. Miners are trusted to perform honest work and pools are trusted to pay them for it. Adding ecash anywhere isn’t gonna change those dynamics because they are baked in. 

Bleeding edge innovation here is by Demand pool and Braidpool, with Demand being much closer to production ready transparent share accounting.