Here's a little trick I just came up with in order to hear how "ambient" my room is.
 
I put on a set of open backed headphones, and listened to a punchy kick/snare combo while adjusting my room monitors volume to the point where I just couldn't hear the direct sound above the headphone sound.
 
Once that was accomplished, anything I heard after the kick/snare hits were obviously in the room, and not in the headphones.
 
I'm not suggesting this is a new trick, or that it's a good way to decide how to treat your room, but it was eye opening for me and gave me an idea of what I need to get rid of/keep with room treatment.
 
I hope others can use this, and I hope that it's not a "Well DUH" tip lol.
 
Gavin

This is probably far more 'Well DUH':
 
Simply producing a loud 'clap' whilst standing in the centre of my room was a massive eye opener.  The whole thing rang like a high-pitched bell for quite some time!
 
Still trying to work out how to mount some treatment on the ceiling, which is that ghastly 'textured/stucco' style thing that should have (well, did) go out of fashion circa 1974.  6" nails keep coming to mind...