There are many square rulers you can use. It is best if you use one that is at least as large (or larger) than your blocks. They all have a diagonal line going through them, corner to corner. Place that line on the diagonal of the block and you will see where it is off. You want to cut all sides so that the block is square - equal on all sides. You will need to reposition the ruler after cutting the first 2 sides.
I wish I could draw this, but hopefully you will be able to understand this.