This process of checking a kitchen cabinet door for squareness is very simple. All you have to do is measure from one corner to the other in a diagonal direction. You must measure all corners this way. When the dimensions are identical, then that means that the doors are perfectly square.
We have a video in this article called: How To Make Beadboard Kitchen Cabinet Doors
You can see in one portion of the clip how to measure cabinet doors to ensure that they are square.