system
Given the following post, respond as if you're commenting on Reddit:
human
[Edge of Tomorrow/Re:Zero kara Hajimeru Isekai Sekatsu] How difficult would it be to tell if someone was reliving a day over and over from the perspective of an outsider?

What are some telltale signs that a person has travelled back in time upon death and are reliving the moments up to said death?
gpt
Sudden shift in personality would be the biggest tell (see RE: Zero), followed by extreme competence or luck. Unfortunately it's hard to distinguish this from mental illness, or other superpowers like precognition, telepathy, probability manipulation, etc.

Even if you are certain, you would have severe difficulty taking advantage of that knowledge. As soon as the person in the time loop becomes aware of you, they can use knowledge from previous timelines against you to avoid you, kill you, or otherwise manipulate you at their leisure.

That said, there are characters in fiction who are observant enough to detect a time traveler and clever enough to provide significant obstacles (see Undertale). Usually they only suspect it, and are very careful not to reveal this knowledge until the time is right.