Check out this new index viewer for Geometry Nodes Fields created by Mohammed Riaz.
A 3D Artist Mohammed Riaz has released a new index viewer for Blender 3.0's Geometry Nodes Fields. This node group lets you view the indices of the vertices, edges, and polygons in the Material Preview/Rendered view. The reason why this node group only works with Blender 3.0 Beta and above is that it uses the new Fields system and it is also outputting attributes to shaders. According to the author, the viewer has a limit of 8 digits and does not require any external data.