A crossline laser level is terrific for squaring quilts! I got one that has three lines, and it worked superbly today (on Incadoodle). It is so much better than trying to use an acrylic ruler to do the job on a piece that is larger than the size of a self-healing mat!
For the first time, I also used a Chaco Liner Pen (blue). It worked very well too. The only caution is to use it on scrap fabric first to make sure the chalk will wash out.