One possibility I suggested yesterday was to have a script that monitors the state of the "known exchanges" steem accounts and alert someone who manages the front ends like steemit if there's no outgoing transfers on one of those account for an hour or so. Then there could just be admin switch they could flip to enable the warning for the exchange without any outgoing transfers. The script could then keep track of the fact that that exchange is probably not functional, and also alert the admins if the exchange does do an outgoing transfer, so that they could re-enable.