What do scientist use to prove a claim or theory?

A successful experiment to validate the hypothesis.

Example:

Many people fell ill after milk freshly squeezed from cows. Their milk also went bad quickly. To counteract this problem: Scientist Louis Pasteur came up with a theory that heat can kill harmful bacteria in milk.

Successful experiment that validates this theory: Heat destroyed the harmful bacteria in milk. When looked under a microscope, milk freshly squeezed from cows contained harmful bacteria compared to milk that had been heated.

Results: People no longer fell sick and pasteurisation was invented to ensure milk stayed fresh and safe to consume.