I square up my table saw’s miter gauge with laser precision. First, I clamp a laser level or pointer to the miter gauge fence and set the gauge to 0°. Then I draw a fine vertical line on my rip fence. I slide the fence over close to the miter gauge and move the gauge until the line “splits” the beam.
Now move and lock the fence as far as it will go away from the miter gauge. If the laser beam is still split by the line, you know the gauge is square. If the beam drifts off the line, the gauge isn’t square. Adjust the miter gauge fence, and repeat the close/far inspections until the beam remains centered on the line. Assuming you are within 1/32″ of centering the beam, your gauge will be accurate to a tenth of a degree.