If you found out your Leader or Immediate Supervisor was doing something against the law, like fraud, what would you do?

Unless you have solid evidence COL Mikel J. Burroughs , my choice of path would be to confront your superior, or consult with co-workers who have observed something similar to what you have. If said superior is giving push back, then I would definitely confront authorities

I had an issue when I was overseas in 1984. Mess cook ALWAYS seemed to have steaks and lobster tails at his home during summer months. While onboard, this once a month meal was limited to E-4's during the summer months. When I realized his nightly cookouts were taken from the ship, I reported it to the ship's Master-at-Arms.

This happen after I got out of the Navy and worked on the civilian side. I worked at a NH and my boss was having an affair with a Commander. So she told me that she looked at his Medical records AND the medical records of his wife and kids. Everyone stopped talking to me after I told my DO. The New DO told me to just work with her. She only got 5 days off no pay. One of those days was Veterans day. I quit. HIPPA is not followed at that NH.

This was a grey area for me. I say this due to the fact of lessons learned from higher. What you see as against the law could very well be saving your life or another.

If there was heavy drugs involved then yes! You have a duty to report but also try to handle it at the lowest level first. Find out how and why.

Fraud, Robbery, Burglary, B&E, etc. Yes report it. Also be smart when you report it too. By that i mean, have your evidence, times, dates, etc. Don't ever just assume anything because as i was told once by my old company first sergeant, "assumptions lead to fuck ups"

After 30+ years of military service, I have seen wrongness committed by Servicemembers, Government and Contractor personnel. You are obligated and required to report. I did report it but didn't mean something happened. But they always get caught!

If it's illegal you are oath and honor bound to report it. Sometimes rules were bent to complete the mission, that's OK. If leaders are stealing to enrich themselves, I'd report it in a second.

Against the law? Or against the UCMJ?
If it's legitimately illegal activity, call the police.
If it's activity contrary to the UCMJ, you can speak to your CC or call your investigation office.

If I were to find out, discover, via rumor mill etc, I believe it is my duty to tell them what was told to me or what I discovered in good council. If they don't address it or it will cause severe harm to others, take a friend with me and tell them and then escalate it up the chain of command.

I gave up trying to report theft or fraudulent activity I would always be told to shut up mind my own business and that anyone with a ranger tab would never do anything illegal. I just sat back until they took things to far and screwed up in front of CID or local police then something would get done but even then the chain would try to minimize the damage

It would depend on your ability to recognize their LIFO and your estimate of ability to speak their language. Company grade Officers don't just walk in to CGs office, so rank matters. Another option is to watch and wait, and stay away. If you would win win based on fact, you have other options within grasp. I would likely not use a hot line. Velvet Glove is effective. Within the organization. Civilians are obviously different.