Scientific discovery relies on high-quality research that is transparent, reproducible, and available for outside scrutiny. Unfortunately, the current reward system often selects for poor quality research by incentivising practices that favour sensationalism, shortcuts, and novelty rather than quality and rigour. Globally, the annual science budget wasted on irreproducible research is estimated in the tens of billions of dollars. Such alarming levels of irreproducibility are not solely due to issues of poor statistical practices, confirmation bias, and selective reporting - various forms of misconduct play large roles. Regardless of the reasons for which misconduct occurs, scientists have a moral and ethical obligation to report it when observed. Here, we present guidance for researchers who suspect or discover scientific misconduct, ranging from the initial stages of fact checking to the later stages of formal investigations. We use our own experiences and lessons learned from conversations with other scientists who have had the unfortunate experience of witnessing fraud. We confirm that whistleblowing is one example of the self-correcting nature of science.