Hammer Go To Brush Dialog
When you've got an invalid brush in your map, compile tools will give you the brush id number of the brush that is bad.
Entity 0, Brush 4, Side 8: has a coplanar plane at (304, -384, 0), texture GENERIC99
To find the invalid brush in your map, you'd just bring up the Go To Brush dialog <+ + > and enter 4 as the brush number, then press OK. The invalid brush will become selected and centered in the Views.
It is also possible to find invalid brushes using the Check for Problems dialog.