This article assumes you have your belts in good tension. If they are too loose, then it will cause the following test to give inaccurate results and diagnose good motors as bad ones.
If it looks like the X/Y coordinates system is getting off during the pattern, usually this is caused by the belts being too tight/loose or damaged, the pulley on the motors on the black motor box may be loose or have something on them, or a motor's phases may not be functioning.
After checking the belts and pulley's you can test the phases of the motors by...
1. Locking both directions on the home screen.
2. Lightly attempt to move the machine in both the left/right and front/back directions. The machine should not move more than 3/16 inch (it is technically 1/6 inch).
3. Unlock both directions and move 1/8 inch to the right and an 1/8 inch toward the front and lock both directions again.
4. Repeat steps 2 & 3 over at least one inch.
If at any time when the machine is locked in both directions it has more freedom of movement than an 3/16 inch (you'll be able to tell if it has more freedom than it should), then at least one of the phases of the motor is not working properly and will need to be replaced. :(