[throw3456548]

Yes, this is really difficult problem. From my experience, doing these three things when being critical puts you in the not an asshole category by the vast vast majority of people: 1. don't raise your voice or yell, 2. don't insult people personally (say "this code is bad" instead of "you are bad for writing this code"), 3. if it is your first time critiquing someone, explain your style of criticism and note that it should not be taken personally.

Of course, there are very sensitive people, and you can't please everyone, but in my experience people will try to be accommodating if you tell them your situation.