Actually, you should be checking for square every step of the process. The most likely place to get out of square is adding borders. In order for the quilt to end up square you should measure the length to cut your borders across the center of the quilt (both length and width) and not along the sides, which are likely to have stretched during construction. You should never just sew a length of border to the side of a quilt and cut off the excess.
If your borders are the lengths they need to be, and your quilt is still not square, you might need to wet the top and block it flat and square before you make your quilt sandwich for quilting.
If you are talking about just cutting a small wall quilt to square, you might be interested in this video:
http://www.youtube.com/watch?v=v-3RIWhBvcA
Hope that answers your question.