As Sandy noted, right now nothing. Tax Notes Federal reported a couple of weeks ago of an upper level IRS employee stating at a conference that the IRS would begin training for ERC examinations beginning next month, looking to roll out exam programs later this year once training is completed. Training would begin with those supervising exams, then move to the agent level.
My guess is that the IRS will do minimal verification to process the 941-X returns (since the whole point is economic relief) but then do a push on exams for after the fact verification. How hard they will push remains to be seen.