How do I know if my square is square?

I'm curious, how can one accurately determine if a given square is truly square? Is there a specific method or tool that should be used to measure its sides and angles to ensure they are all equal? I'm looking for a reliable way to verify the squareness of a square, whether it's for a construction project, a mathematical exercise, or simply out of curiosity. Can you provide some guidance on the best practices for assessing the squareness of a square?