If your boss says no, or that’s not your job, let it drop and polish up your résumé! If your boss says yes, start with something pretty small and benign and see how they react. If they react well and reward the candor, keep going. If they don’t, give up immediately or assume ill intent. Try again, carefully, but if you get the same reaction the next time, it may be time to move on. You deserve a better boss.