FINRA requires retail brokers to make a report on price improvements based on a retroactive analysis vs NBBO. So, yes, they do know what NBBO was, and whether

FINRA requires retail brokers to make a report on price improvements based on a retroactive analysis vs NBBO. So, yes, they do know what NBBO was, and whether they were truly price improvements or not, and that's being monitored and tracked, and the brokers get yelled at if they violate that. Pretty much all conspiratorial, "motivational" analysis of PFOF critics on HN are simply wrong. As Matt Levine explains it clearly, from the utilitarian perspective, PFOF is not a bad thing at all for retail investors. Whether it can be even better is a different question and clearly SEC is trying to find that with the new proposal, but as is always the case with a complex system with differently motivated actors, what exactly the new balance point for the new system will be is uncertain.