codesign -dv --verbose=4 /path/to/InstallESD.dmg
Apple maintains a hidden catalog of all macOS installers. This is the most method outside of the App Store. download mac os high sierra 10136 dmg verified
High Sierra hasn’t received security patches since late 2020. Never use it as a daily driver connected to the internet without a firewall and updated browsers (Firefox ESR still supports 10.13). codesign -dv --verbose=4 /path/to/InstallESD