Hey John,
I agree, a saw table usually is a good reference 
If you are sure that 1. your both Y assemblies are with no twist in their feet and 2. they sit on a highly coplanar surface, i.e. there is no feet higher than the others (if not, check with the fishing line method or with winding sticks), then what remains would be to put these X hollow shafts parallel and one over the other by the tramming bolts.
If you have a different tilt of the router axle when X is at left than when it is on the right of X travel, you could attach one vertical ruler that touches both hollow shafts on each end of the X axis. Each ruler would prolongate what you want to see in the direction of ceiling. Then you would sight from the side if both rulers are parallel or how much they deviate from being parallel. This is a workflow similar to using winding sticks: By prolongating the contact line between the lower und the upper tube using a ruler or a winding stick, the deviation between the tubes at left X end in comparison to right X end is multiplied (exaggerated) and with this, the tilt (or twist) becomes more visible. Cabinetmakers and Joiners used winding sticks for centuries (millenaries!) as method to eliminate twisting (see a few links below). I don’t know if you used that method already but I can say it is very effective and often underestimated. And the same way that it works for ensuring the coplanarity of a piece of wood or a table surface (with winding sticks large enough), you can use this method to ensure that your two hollow shafts on X axis are one over the other at both ends, left and right, by using the rulers as “vertical” winding sticks. I would make a drawing if I had more time but I hope you can imagine?
Note that in this step, it is not at first essential that to eliminate any tilt, just the difference between tilt on left X end and tilt on right X end should be eliminated. The general tilting that remains same over entire X travel could then, if possible, be eliminated either by using the tramming bolts with exactly counting every revolution on each side and appliying it identically on both sides or, if tramming bolts come to their ends, by shims under the Z slider, as @SkyKam mentioned here.
Further Reading
About Winding Sticks
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Liam Rickerby, CC BY-SA 4.0, via Wikimedia Commons |
How To Use Winding Sticks - Youtube
How to make Winding Sticks (Part 1) - Paul Sellers - Youtube