How can I determine whether a home with common walls has good soundproofing?
While codes and regulations can be good, there may have been a bad builder or contractor in one particular house. Or an inexperienced homeowner making repairs / upgrades. So, regulations can be a good guideline, but I wouldn't treat them as gospel.

Your best bet is to either try to hear the neighbors as they go about their daily business, or go talk to them and ask how good the sound-proofing is. I'd probably do both if I really liked the house. Both require the neighbors to be home, of course, so you may need a second visit when they're home. Note that if your direct neighbors aren't home on your first visit, other neighbors may be around who could offer opinions on their houses. Not directly comparable (see first paragraph), but it's still informative.