Possible Clues:

• 'I shoulda known that!'
• Moron's comment
• ''Thank you, Captain Obvious!''
• 'That's obvious, stupid!'
• 'Well, ___!'
• 'Any idiot knows that!'
• "That's so obvious!"
• ''It's so obvious!''
• Forehead slapper's exclamation
• Comment made while slapping the forehead