Correct. That certainly works. But you need an angle-measuring device to measure the angles and see if they are the same. You can compare all four sides of the quadrilateral as well as the diagonals with only a length-measuring device. And since you are only comparing them for equality, it doesn't even have to be accurate (a piece of string etc.).

I don't even have a square around here, but I certainly have something I can mark with a pencil to compare lengths with.