The mirror method seems the simplest and most practical, but there is one potential problem. Your pointer towards the lens, no matter how it is arranged has to be exactly flat on the subject plane. That means it should have a large base of contact. Otherise slight errors could be magnified. Thus carpenters don't rely on T-squares when checking such matters for large surfaces. They use various simple tricks from geometry.
Here are some other things you might do to get around that problem.
If it is feasible, put a large rectangle centered in the field of view on the subject plane. Then measure the distances to the corners from the center of the lens. These should be very close to equal. (If you want me to, I can calculate how much the difference in lengths translates to in terms of a displacement from parellism.) A related method which could be used to check the miror or laser pointer method would be to measure the distance from the center of the subject to the lens along what you've established as a right angle, measure the distance in the plane to some reference point, and then measure the distance from that reference point to the center of the lens. The Pythagorean Theorem tells us that the square of the diagonal measurement should be equal to the sum of the squares of the other two measurements. Carpenters use this method by choosing dimensions so the triangle should be a 3. 4. 5 right triangle.
If the subject plane is vertical, once you have the rear standard leveled, you can also check the lengths of diagonals of the image of such a rectangle in the subject plane. If these are essentially indistinguishable using a scale in mm, and the rectangle comes close to filling the frame, it would seem to me that you would be close enough for all practical purposes. If the subject plane is too far away, you could tape a tape measure or other ruler to it in the center and in the corners and measure the lengths on the gg of equal lengths in the subject. These should be very close to equal.
Another related thing to check is that your standards are as close to parallel as possible. There is often some play in the default positions of the standards, and they can be slightly out of parallel. The easiest way to do this is to turn the tripod head so the camera is pointing vertically downward, using a level on the rail as a guide to see if it is plumb. Then check to see how close the standards are to horizontal with a level. Check on the gg for the rear level and on the front of the lens barrel for the front standard. For wide angle lenses, small displacements from parallelism of the standards can tilt the exact subject plane enough to disturb focusing in the corners or edges.