They do this all the time, I'm using ChatGPT in Instant mode and it auto updates to thinking if my question is complex. Most of the time this works.
To answer your question: A large language model itself does not know this (afaik). But chatbots are not "just LLMs" but a whole bunch of systems (and models) around them.
To answer your question: A large language model itself does not know this (afaik). But chatbots are not "just LLMs" but a whole bunch of systems (and models) around them.