For those of you who sit at home analysing other's mixes, this might become useful.

Today, by sheer accident, I've put Nirvana's Nevermind through Sound Forge's "sum to mono - use difference between channels". It is an extremely powerful mix analysis tool - you can clearly hear reverbs (kick/snare/vocals/guitars in the case of Nevermind), hear how the kick and snare get completely different treatment at different sections of the song, and conclude lots of information regarding the stereo usage.

You can also achieve this by phase inverting one of the mix buses and pan both hard centre.

Try it out.