How to prove episodes of academic misconduct?
If a professor wants to falsify data now, the professor probably falsified data in the past.  Look through previous publications by that professor for data that is obviously falsified.
Plagiarized images are the most commonly detected form of fabrication.  Not all image reuse is falsified data, but if two publications present the same image data as if they were recorded separately, this is proof of falsification.
People who falsify data are likely to engage in other academic misconduct, such as plagiarism.