An easy check for your belt tension is to grab one of the carriages, and move it up and down on the tower quickly. If you hear or feel the belt skipping, it's probably too loose.

To tighten it, you'll need to remove the side panels on the top of the printer. Inside above the towers are the motor mounts. With the screws loosened, you can rotate them clockwise to tighten the belt.

If you can't tighten them enough, leave the screws loose, remove one belt clip from the carriage on that tower, pull the belt tight, and reinstall the belt clip. Then you should be able to tighten the belt enough by rotating the motor, then tightening the motor screws.