Waila Harvestability is a client-side add-on for Waila mod 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.
While holding an Iron Pickaxe (with default settings):
Silk Touchability and Shearability:
Waila configuration screen: