You won’t be able to tell every boss that they’re wrong, and even a boss that is normally accepting of this kind of feedback may not be open to it at every point in time. With that in mind, understand your boss’s personality and work priorities. If they care deeply about impressing their own boss, for example, it may be unwise to levy an unexpected criticism when the two are having a conversation in the hallway. Wait for a private moment when they'll be expecting feedback.