Another possible tool would be some kind of mediation-tool provided. Pool users could send in complain-reports for f.e. pending/missing payouts which are always public visible. The pool owner will get a notice of this in order to give a statement on this. If there is no response within a given time by the pool owner this could be another negative indicator for trustiness.
I prefer this idea over the other one (1st idea), although there's a small issue (some pool owners might respond late) so a few more steps are needed.
Step 1: Pool users report the issue (with links to their payout addresses)
Step 2: Verification stage (to see if they've reached the threshold and also to check the missing payout)
Step 3: Verified reports will be added to the website (with live status updates - either monitored by someone or some sort of automated process) then pool owners, automatically receive an email and etc...
Live status updates:
Red status update = pools having payout issues
Green status update = pools without a payout issue