We couldn't know. Humans mimick patterns. The claims that LLMs aren't smart because they don't generate anything new fall completely flat for me. If you look back far enough most humans generate nothing new. For example, even novel ideas like Einstein's theory of relativity are re-iterations of existing ideas. If you want to be pedantic, one can trace back the majority of ideas, claim that each incremental step was 'not novel, but just recollection' and then make the egregious claim that humanity has invented nothing.

> But if it does happen some day, how will we know? What are the chances that the first sentient AI will be accused of just mimicking patterns?

Leaving questions of sentience aside (since we don't even really know what that is) and focusing on intelligence, the truth is that we will probably not know until many decades latel.