You absolutely can judge other people's skill despite never attempting something before. Have you ever judged the verdict of a referee? Or a song? Or a video game? Or the food that somebody else has made? Or the build quality of something you are purchasing? Judging other people's works is something literally everybody does. Saying you can't do it is just not true. Now saying that we shouldn't judge other people when we haven't attempted it ourselves would be understandable, but that wasn't the exact question that was asked here.