On Zettlr’s GitHub page, you can navigate to the Releases-section. Find the release you want to install. In that release’s assets section, you can find both the binaries as well as the SHA256 checksums. You can download both files, and use a tool such as sha256sum to verify that the binary matches the checksum.
Note: If you update the app via the updater, Zettlr will automatically check those checksums for you. Package managers such as Homebrew or APT usually do the same.