Modding Help Detect if mod is installed server side (for vanilla multiplayer porting)?

Hi, we've been working on creating vanilla multiplayer compatible versions of our mods.

I noticed that Vanilla servers would boot your client for so much as holding an item without an id in their registry or trying to set the visuals of a held item to an image address that is not in their files.

To port effectively, we want to detect whether the server has a given mod installed or not (which it always is in single-player).
If the mod is installed server-side, we'll use custom images and custom projectiles; if not, we'll use vanilla items with modified parameters and vanilla visual assets.

Thanks!