I use a square ruler for the corners and a ruler along the edges. Find a seam such as the border, and put the "correct" line on it and check to see if it needs to be trimmed. Some of the edge may not need trimmed. The square ruler will show if corners are square. I square up my blocks as I make them but sometimes the quilting will make things wonky. I do this in a small area and even trim/square up king size.
Hope this helps.
D