The archive is the BepInEx package itself.
Contains at least one file besides marker files.
Contains a .dll file, which only new-engine mods have.
Contains a .js file, which only old-engine mods have.
The marker files the archive contains.
The archive is the MelonLoader package itself.
A path other than a marker file mentions BepInEx.
A path other than a marker file mentions MelonLoader.
A top-level entry is part of the old engine's folder layout, so the old-engine path repair can place it.
What an archive's file list reveals about the mod inside it.