QC gets a report of every loss of standard separation. They look at the radar data and the audio and determine if there was indeed a loss, or if there was another form of separation (like a pilot was maintaining visual, etc..). I don't know if they still use TARP downstairs or some other system.
The example i gave happened to a controller here. She had two aircraft on divergent headings (20 degrees), but their ground tracks diverged by less than 15 degrees for a short time and an mandatory occurrence report was generated and the loss was investigated. I don't know what, if anything, happened to the pilots.