Nobody can tell you if their criticism is sincere or authentic. You, as the criticized person should decide on that. Personally speaking, I can almost easily figure out if any criticism directed at me is real or not. I don’t know whether others have the same natural ability or not. I should also add that I generally prefer polite criticism to compliments, as I’m always ready and eager to learn from my mistakes.
What’s more, the setting in which you are criticized carries some weight. I mean whether you are criticized in public or in private. A real friend will never criticize you in public as it might ruin your reputation.