Tried this out last night and found and corrected quite a bit of error! I knew my enlarger was off a little, but it was more than I expected.
I'd been eyeing one of those nice laser alignment boxes for the purpose but couldn't justify the bucks for something that'll get used so infrequently.
I've got into it $30 for a David White 48-3BTORP TRI-Beam Three Beam laser level. 260590865011 on ebay.
This is a solid metal based laser level tough enough to beat someone with; quite a bit different than the plastic black and decker sort of thing you might get for painting in your house. The laser is also fixed and not on any sort of balance or moving counterweight.
This is with my Omega Chromega B. I removed the lens and raised the head and removed the negative carrier. Lacking a small flat mirror, I took a large polarizer filter I had and set it where the negative carrier would go. Any filter would do. A filter seems to work best.
Set the laser on the baseboard with the vertical beam going up through the lens hole. You'll see the beam on the filter as some of it goes through. Some of it will reflect back down the baseboard. Ideally it should go right back into the laser. Mine was off by about an inch. Rotate the laser 90 degrees (like a clock hand) on the baseboard and check again. It should be the same or your baseboard is warped or the level isn't sitting flat.
Loosen the alignment adjustment screws a little on the enlarger, nudge things into alignment so the laser goes straight back to it's source. Tighten up, and you're done. Remove filter, reinstall lens.