use spirit level to check walls and counter
a carpenter square tells if cabinets are square
Check the Cabinets
Check the cabinets to make sure they are level and square. Also check the walls to make sure they are straight.
A spirit level will show if the wall is straight and the counter is level (Image 1).
To tell if the cabinets meet at a 90-degree angle, put a carpenter's square at the apex of the two 90-degree corners (Image 2), and if the cabinets are flat against both sides of the square, the cabinets are square to one another.