Hey Terry,
the fishing line method is one of the methods for checking if your machine is twisted. With a twisted machine (which is usually caused by a twisted table or tabletop), your Z axis has not the same vertical angle when the X gantry is on the right than if it is on the left. But it may show up perpendicular to the part of the tabletop that is below it when you measure it with a square or with the surfacing bit ridges method, which can be confusing.
So beside checking for perpendicularity of Z axis to tabletop, often also called “tramming the router”, it is also necessary to check if your machine is twisted. People who have a table with casters and no torsion box use casters with height adjustment, and check for twist every time they moved their table and adjust the height of the casters. The same is done by using the Any Surface Leveling System in case you have the QCW Frame.
You can use the fishing line method to check the machine for twist by placing the lines diagonally over the tubes, from upper Y tube on one side to other upper Y tube on the other side (move X gantry fully to one side first), or you can use this method to check the coplanarity of the corners of your table without the machine. Remember that if the two lines touch each other at their crossing point in the center, you got to swap the two lines so the other is on top. There must not be a gap either, in the end they have to touch each other in both dispositions, to finally be sure that the four corners are in one plane (coplanarity achieved)
Another possible cause of one Y rail being higher than the other, or of a twisted machine, can be if one rail is twisted itself (video). Onefinity assemble their rails on a flat granite table, but sometimes, twist on a single rail can happen during shipping.
Finally to come to and end with enumerating what generally can be checked, but what seems not to be an issue with your machine, is if the X rails are on top of the other (and this over their entire length). This method using vertical winding sticks can show you whether your two X tubes are aligned to each other.