I'm chiming in. This may sound terse but hind-sight is 20/20.
At your earliest convenience, find a building/room/office where there are firewalls between the 'units'. The walls may be cement block or even brick. Or, at worst, find a unit where the walls are ALREADY soundproofed.
You can bang and pound on the walls and tell if there is block or brick between them. As for testing a non-block/brick wall for soundproofness, you need to have someon on the other side of the wall make some noise. Do the test early in the morning or in the evening when the outside 'noise' is more dimished. Saturday in the AM is a good time to do the noise-test.
Steve's point about retrofitting is obviously correct. It's not a simple matter of just 'hanging' something up on the walls. Would you consider Maxwell Smart's Cone-Of-Silence??! My best to ya and hope this helps.