wav3front top would be my first go-to, that should show pretty clearly which accounts are screwing the pooch.
There's lots of options for server monitoring that will store historical data for troubleshooting too. I like using Linode's Longview service since it's low cost, low footprint, and dead simple to setup. You don't have to use Linode to use Longview, you can just make an account and use the monitoring service, that's what I do (though I will spin up a server for testing sometimes, or use it in an emergency if needed).
