Enable visual alerts for system messages

A quick tip to enable visual alerts when a system event occurs (for instance if you have your sound off):

Go to 'System Preferences', then 'Universal Access'. Click the 'Hearing' tab, then check the box for 'Flash the screen when an alert sound occurs'. Now, whenever you get an on-screen message like a popup box or notification of some sort, the screen will briefly flash, letting you know something has happened.

universal access