This fixture is one that is used almost every day.  The Dial Indicator has a long shaft that can slide forward and back to adjust spacing from the table saw slot.  This is used to align the slot to the blade and the fence to the slot.  This use found a very noticeable wavy condition in one of my auxiliary fences for the Unifence.  This was a after market fence that had a 1/2" thick UHMW replaceable face.  The problem was this was attached with four countersunk screws and the UHMW was not flat.  This surface was wavy up to .015" between screws.  I added several other screws and some tape shims and now it is parallel to the slot within .003.

I have also found this fixture to be invaluable when trying to make small adjustments to the fence when finessing a cut.  If you need to make the cut just slightly smaller or larger it is sometimes difficult to release the fence and reposition with any precision.  Put the fixture in one of the slots and adjust the indicator back/forth till it makes contact with the fence and zero out the indicator.  Now you have a reference point and can move the fence any increments you wish.  If you have a large cut and the indicator will not reach the fence you can add a scrap spacer, as long as you do not shift the spacer it can be any scrap from the wood pile.

The Jig is simple to build, the vertical leg needs to have 3/8 dia. holes drilled at matching points that you want to check squareness.  I just went around positioning the veridical leg against the different tools in the shop and marked the hole locations.  Next you need to drill the 3/8 holes with a good brad point drill or Forstner bit.  They need to fit the stem of the indicator with minimal play.  The vertical leg is now split down the center on the ban saw or with a fine tooth saw.  Find a spot between holes near the 1/2 way point and drill and tap for a 10-32 thump screw, optional way is to drill through and use a nut.  The vertical can now be glued or screwed to a base.  The bottom of the base is relived to provide three points on contact, two in the front and one in the rear.  Directly above the front pads add two round head screws for vertical reference points

The fixture is calibrated/set by the method of rotation to eliminate error.  You do need a plate that the fixture can be compared to.  The only requirement is the plate must be the same thickness at the contact points.  Any difference in the thickness will show up as squareness error.  The plate used here is a extra blank Phenolic router insert from WoodPeckers that I have measured for thickness and found to be less than 0.001.  The other requirement is to have a flat surface to use as a horizontal reference.  For most of us woodworkers we do not have a Granite Surface plate in out shop so we must improvise and use the flattest surface we have which for most people will be the Table Saw.  Position the Vertical plate so the reference pads are not spanning the area were the wings attach.  With fixture in Position #1 set indicator to Zero, now rotate to Position #2 and record indicator reading.  The actual vertical squareness error is 1/2 of the total error observed in position #2.  Set the indicator to 1/2 the error and recheck position #1.  The indicator should show the same amount but in the opposite direction.  The squareness comparator is now set and can be used to check other squares or machinery.  In the third picture I have used a 3/8 parallel to bridge the base contacts, the indicator was preset at 3/8 inch when doing the original set-up.