full bed level
thingiverse
Most useful for checking of the bed is warped. If the corners are fine but the middle is off the bed is warped.
Download Model from thingiverse
With this file you will be able to print full bed level with your 3D printer. Click on the button and save the file on your computer to work, edit or customize your design. You can also find more 3D designs for printers on full bed level.