After asking the question "Do you like it?" during a critique, even if your answer is "no" you should point out details that you do actually like about the piece. This is especially important when giving constructive criticism to someone who is looking to improve their work, mostly for the sake of kindness. When doing this at a gallery, it's not as important though it still helps to validate your opinion if you can acknowledge things that are actually decent about the piece even if you don't actually like it. You should first explain what you like about the piece's technicalities(objective things such as tone, value, anatomy, perspective etc.) and then you should explain what you like about the piece's creativity(subjective things such as aesthetic, subject-matter, brush strokes, & symbolism). Next, you should get into the actual criticism, explaining what you don't like about the piece even if you do like it overall. You should, similarly to when explaining what you like about a piece, first explain what you dislike about the piece's technicalities and then explain what you dislike about the piece's creativity. Especially when judging technical ability(even sometimes creative ability), critique can be particularly difficult when judging a master's work..it can even be difficult when trying to critique someone of similar or slightly higher skill than that of yourself. There are, of course, many different methods to go about critiquing artwork, and there isn't really one right answer.