I agree to trying to pull the top into square first. It is a technique that can be surprisingly effective. I find folding the top to see if it's square is rarely accurate as there are seams and bulk that will throw it off. If you are going to look for square by folding, do use the pins to secure your seams so they don't move and give a false reading.
I find laying the top flat and measuring it, as Mad Quilter suggested, to be the most effective way.