I have jigs, calipers, and dials for this fairly regular job of aligning the table saw blade to the miter slot.
However, for the armchair engineers out there, throw rocks at the following suggestion.
Lets say you remove the blade, loosen what you need to loosen (either the table nuts or the trunions depending on type). Raise the arbor to the top of its travel.
And you have a device which fits on the arbor like a blade (maybe thicker for more rigidity) but it is shaped like an L with a miter bar at the end of the L. Then you would lower the arbor slowly with everything loose until the miter bar is in the slot. Then you would have everything perfectly square and you lock it all down.
Why wouldn't that work?