How to Tell Whether SuperMartijn642's Core Lib Is Actually Installed and Working

Core Lib adds no items, blocks, or recipes, so you cannot confirm it by looking in your inventory. You confirm it indirectly, through the mods that depend on it.

Check the mod list

Open the mods screen from the main menu and look for the Core Lib entry. If it is listed, the library loaded. If it is missing entirely, none of the dependent mods will have loaded either, and you will usually see a crash report or a red error screen naming the missing dependency.

Check a dependent mod's content

Pick any installed mod that requires Core Lib and open one of its GUIs — a machine, a storage block, a config screen. Core Lib supplies the shared GUI, block, tile entity, and networking code those screens run on. If the interface opens, renders, and responds to clicks, the library is functioning. If a dependent mod's blocks are missing from the creative menu or its machines refuse to open, the library is the first thing to suspect.

Read the log

On startup, the log lists loaded mods and any dependency resolution. A missing or mismatched library shows up there as an unresolved dependency rather than as a gameplay symptom. When in doubt, remove the dependent mod and re-add it; the loader will re-report what it needs.