I agree that it's become rare for anyone to knock other than salespeople, political canvassers, delivery guys, or missionaries, so definitely expect people to be a little wary when answering (if they even answer at all). However, I think once you mention the HOA, they're likely to talk your ear off.
Try something like this:
"Hi, I'm sorry to bother you but I was thinking of moving into unit , just down the hall from you, and I'd like to know how you feel about your HOA? The dues seem pretty OK, but you know how it is -- you never know if you're going to get what you pay for with these things."
Some people might say they don't have time, and others might be too suspicious to give an honest response, but I think any truly negative responses will be rare. Instead you'll find people enjoy giving you their opinion, once you reassure them that there is no other agenda. There seems to be this basic human need to be listened to and validated, even if only by a complete stranger.
On top of this, I think one hallmark of a good community is a willingness to answer the door and talk with (potential) neighbors. If you find most people aren't willing to chat at all, then that's already a pretty obvious red flag.