Correct, it is not possible to prove. If Magnus had anything - anything - other than "I felt this way" and "he seems too chill" I would suspect something. Those things could be:
- Hans was walking weird (something in his shoe)
- He was making weird movements
- He was distracted, or similar, indicating he's messing with some device
etc... then sure.
Magnus did not say these things, and that is telling.
What I believe happened is Magnus (someone who has presented a lot of anxiety in the past - see this video: https://www.youtube.com/watch?v=WR-4_ouXUV4 but easy to find other examples) was really nervous that Hans may be cheating, and that impacted Magnus's play (he played a very poor game).