The following instructable details 2 ways to find out if something is square, 1 way to draw an accurate perpendicular line, and 1 way to draw an accurate parallel line.
These tricks involve virtually no math to do and are scalable to any dimensions from millimetres, to miles.
The 4 tricks are probably not something you will use every day but are really handy to know and will allow you to amaze others with your skill and mental prowess.
I have embedded the video podcast of this because it is sometimes easier to see it done than to try to understand a written description. my part is at 5:24 Don't be shy about watching my other episodes or checking out my other instructables.