This mod is a simple utility that allows you to query which items can be placed in a specific slot by holding down Left-Alt. You can also find this mod on CurseForge or Modrinth.
This is particularly useful in modded environments where it may not be clear what items can be placed in a modded GUI.
Relevant items can also be quickly transferred from your inventory by clicking on them in the query window:
This mod works client-only, so it does not need to be installed on the server to run!