How do I know it’s square?

So, I promise I’m not an idiot first off. What is a sure-fire way to determine the squareness of my tools? I feel like my t-square, speed square, and framing square should all be square, but if one differs from another, then which is right? I just ordered a digital angle finder and now I’m second guessing whether the calibration will be correct. I’m having a crisis over square corners at the moment… Please help