Checking the Mod's Description
Most mod pages list their dependencies in a dedicated section. If Fabric API appears there, the mod will not work without it. Some mods bundle their own libraries and do not need Fabric API at all, while others list it as optional because only certain features rely on it. Read the description carefully rather than assuming every Fabric mod requires the library.
Looking at the Jar Itself
If you already downloaded a mod jar, you can inspect it without launching the game. Rename the file to end in .zip and open it with an archive tool. Inside, look for a metadata file that lists dependencies. If you see an entry referencing Fabric API or its module names, the mod depends on it. This is a quick way to check a whole folder of mods before committing to a play session.
Testing in a Throwaway Profile
Create a separate launcher profile with only the mod in question and Fabric API installed. Launch it. If the game loads and the mod's features appear, the dependency is satisfied. If it crashes with a missing-dependency message, you know Fabric API is required and possibly the wrong version. Keeping a test profile separate from your main world prevents a bad mod from corrupting a save you care about.
