>>12273667Get an object, such as a blue stone, that you definitively know exists. Ask the person in question to move it.
If the object is moved, either you have proof of telekinesis, or the person exists. Applying occam's razor: it is probably that the person exists.
>How can I know for sure blue stone exists?See above: have it interact with surroundings you know exist.
>How can I know for sure surroundings exist?You can't.