The 2 screws at the bottom of the X carriage are how you can make minor adjustments. I made mine square by clamping the edge of a framing square to the spindle mount before tightening the 2 screws. I also squared mine up front to back by sticking business card shims in between the Z makerslide and the X carriage.
You can check for out of square by doing a large pocket with a large bit, like a 3/4" surfacing bit. Any out of square will be magnified and make those ridges, so you can get it almost perfect.