The simplest tool is to take a piece of over-exposed and developed film (i.e. as black as it can be) and scratch a grid of fine lines right across it on the emulsion side using a pin or the tip of a knife. Put this in the negative carrier (emulsion down) with the lens at full aperture, and try to focus all the lines at the same time. The lines will be much easier to see and to focus crisply than the details of a normal negative. This should give you a clue as to which direction things need to be straightened. I had to insert shims of cardboard (from a breakfast cereal packet) under one edge of my negative carrier, and after a long trial period I finally glued them (two thicknesses) in place, where they have been for about 20 years. Note that - depending on your lens - the outer field may not focus crisply at full aperture, and you may not be able to see whether this improves when the lens is stopped down. So ultimately you will need to do a test print of your scratched grid with the lens stopped down.
If you want more technical tool, Ralph Lambrecht describes how to make a laser alignment tool (similar to the commercial one in Greg's post #7) in his book Way Beyond Monochrome. I tried to do this, but failed - you really need precision metal-working machinery to align the laser in the first place! I think I managed fine without it anyway.
I don't know Beseler enlargers, but I have the impression that they are quite well engineered. My experience with a lesser enlarger was that even when the thing was aligned at a moderate height, it was out of alignment again if the head was raised to the top of the column, because the column drooped under the weight. In the end I dispensed with the enlarger baseboard, bolted my enlarger foot directly to the bench, and fastened the top of the column to the wall via a threaded rod, allowing adjustment until everything was aligned and rigid. That solved my problem, but with luck you won't need to do this!