A client-side add-on for
Waila that adds information about the harvestability of what you are looking at.
By default, it shows whether or not you can currently harvest the block, the required harvest level of the block, and the tool type that is effective against the block. Plus, there are many configuration options in the Waila modules configuration screen to tweak it to show exactly what you want it to.
Note: This mod is mostly intended for use with Tinkers' Construct, but the information is still applicable without it.
The idea for the mod was suggested by
DraxisWuf.
Screenshots
While holding an Iron Pickaxe (with default settings):
Waila configuration screen (default settings shown):
Minimal mode: