Here's a run-through of what information you can see for specific entities:

• Most entities: Toggling with Middle Mouse will switch the entity flag between On/Off and Open/Closed respectively.
  • Toggling while holding [SHIFT] switches between locked/unlocked.
• Sleeping bag: Shows person who's been Assigned To it
• Quarry/Pumpjack: Shows the resource static type it produces.
  • Toggling will iterate through all static types and update it in real time.
• Doors: Shows the code and guest code number if there's a lock on it (when Show Extra is enabled).
• IO Entity: Shows the current energy processed by the entity.
• Planter Boxes: Shows the temperature of the growable plants in it in Celsius and Fahrenheit.
• Modular Cars: Shows the code if there's a lock on it (when Show Extra is enabled).
• Vending Machine: Toggling will flip the vending machine.
• Quarry Engine Switch: Toggling turn the quarry switch on/off.
• Building Blocks: Blocks that can be rotated will rotate upon toggling.
  • Enabling Extra Settings on the UI, will allow you to destroy an entire building.
  • Hitting a building block with the hammer, will repair all entities inside of the building.
  • Deselect the Hammer slot to cancel both batched building destruction or the repairing process.

By default, you need to have Creative Mode enabled on your player, you can do so using togglecreativemodeuser myname 1.

To activate the UI, you must hold a Hammer or Garry's Mod Tool. To use the Hammer UI without having Creative Mode enabled, players with moderator and/or admins can run hammer creativebypass 1 to bypass that behaviour. To use the UI without holding a hammer altogether, you can run hammer hammerbypass 1.

Here's the entire source code: HammerModule