If you're using a tool, lets say an iron axe, and you break a block of wood, above your experience bar it will say "Iron Axe" as if you just switched to it.
Steps to reproduce: Grab any tool or weapon of your choosing. (except for a bow.)
2. Mine any block that takes time to break. Dirt or leaves or something like that will work, but not something like grass or fire.
3. You should see white letters of your weapon appear above the xp bar.