ChatGPT does have some feedback that can be used to evaluate, in the form of thumbs up/down buttons, which probably nobody uses, and positive/negative responses to its messages. People often say "thanks" or "perfect!" in responses, including very smart people who frequent here.
ChatGPT was trained (in an additional step to supervised learning of the base LLM) with reinforcement learning from human feedback (RLHF) where some contractors were presented with two LLM output to the same prompt and they had to decide, which one is better. This was a core ingredient to the performance of the system.
