How can I verify that a RAT is gone after reinstalling Windows?

Malware can theoretically survive in unusual places such as compromised firmware or a bootkit, but that is far beyond what you would normally expect from a simple joke RAT. If you erased the drive rather than merely reinstalling Windows over the existing files, the practical risk is extremely low. Firmware persistence would usually require advanced skills, a specific vulnerability, and a reason to target you.

A proper clean installation that deletes the existing Windows partitions and installs from trusted media should remove an ordinary RAT. The programming language does not make it invisible; antivirus tools detect behavior and known malware patterns, not just a particular language. After reinstalling, fully update Windows, install firmware and driver updates from the laptop manufacturer, and run a reputable security scan. Also change your important passwords from a separate trusted device and enable multifactor authentication, especially for email and banking.

Keep in mind that a Windows “reset” and a true clean install are not always the same. For maximum confidence, use a trusted computer to create official installation media, boot the laptop from it, remove the old system partitions during setup, and install Windows fresh. Once configured, check startup apps, running processes, browser extensions, installed remote-access software, and unusual network connections. If those look normal and scans remain clean, there is no practical reason to keep reinstalling.

You can’t mathematically prove that a negative is impossible, but security decisions are based on evidence and reasonable threat models. A clean reinstall followed by normal file, process, and network checks is strong evidence that the RAT is gone. If your accounts show no suspicious activity and the person who installed it is no longer involved, repeatedly wiping the laptop is unlikely to provide additional protection.