Calibration axis
thingiverse
To check the squareness of the printed object : print it and test it with a right metal bracket. If you're not satisfied with the squareness your printer has, you can tweak it by using "Bed skew compensation".
Download Model from thingiverse
With this file you will be able to print Calibration axis 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 Calibration axis.