Well there are two methods.
1) use a trianglular square and fit it in each corner and see if it is flush. This will require a long one to get out there.
2) use the 3 4 5 concept that people use when stringing out a deck or patio layout.
From the center of the 90Degree, measure out three feet in one direction, then four feet in the other direction and then from the end of the 3 and the end of the 4 the distance should be 5.
This method is pretty accurate, but you are using possibly unsound walls, meaning they may not be traveling a straight path.
Then you could purchase one of those nice lazer levels. This would really tell accurately and precise. But, do you have $150 just to tell squareness.
I will tell you that right now, without seeing it that it is not square. No home, $50K to $3 mil, has square walls, just not possible nowdays.
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