Perhaps it helps if you define "impolite" as "status-grabby". Thus when someone says "nice" things in what comes across as a condescending tone it can be recognized as impolite on that basis -- regardless of their intent.

It's a relativistic criteria though: a given statement can offend some but not others. As an example, the degree of technical explanation afforded for a complex topic. If you put in too much, the experts feel like they are being condescended to. If you put in too little, the less trained feel excluded because they cannot follow all the jargon enough to relate it to anything they know.

Perhaps the real cheat code in this case would be the skill of writing things in a manner that people can interpret into their own preferred range.