Just thought I would share a simple aid to squaring your enlarger head. I know some use lasers and mirors, which would require complex squaring of the laser in its mount etc, cost....
Try this. It is SO SIMPLE AND WORKS EXCEPTIONALLY WELL. It takes far longer to explain than to do and is almost requires no cost if you have a computer printer and some OHP film sheets for it:
Step 1: Ensure your easel is level using GOOD QUALITY SPIRIT LEVEL - preferably a longish one for max accuracy. (Cheap ones are usually rubbish - you will find those cheapies in the shop do not agree with each other!). Be very careful with this levelling as it is critical. I use simple pieces of card as wedges to level my 20x16 easel.
step 2: Using transparency film (for OHPs) print off a black pattern (concentric rectangles (or squares for 6x6) with thinish lines works well) which is slightly smaller than your neg image area using an inkjet or similar at the LOWEST print resolution. High res will produce a smooth line which is not what we want. You will find that (well with my epson 790 at economy/draft B&W for text) that the line is not continuous but made up of sharp lines in the direction that the head applies ink, like scratches. As it is at low res these lines have a tiny gap but themselves are very sharp edged. This means that you have a sharp textured series of concentric boxes. Use this in place of the neg preferably in a glass carrier (which will keep it perfectly flat) and project onto the level easel starting with a wide aperture for minimum depth of field. It will be blindingly obvious if the sharp dashes are sharp in one plane but not another (e.g this will appear as sharp on the left but getting softer progressively towards the right etc or if you are unlucky sharp in one corner but fuzzy elsewhere in hich case you are out in 2 planes). This can be done with the naked eye, but a grain magnifier provides big sharp lines that are either pin sharp or have fuzzy edges and aid fine tuning.
Step 3: Adjust enlarger head until sharp everywhere....assuming it can be adjusted!
Keep this alignment neg on file and use to check when you have doubts about alignment. This takes all of about 1 minute. You will be able to see clearly that if you have imperfect but close to perfect alignment, this may be obvious at f4, but invisible at f11 at great enlargement but invisible at both apertures on a tiny enlargement. It helped me find out in a jiffy what the limitations of my set up were. One enlarger I have I cannot get perfect at open aperture and large enlargement, but I realised it was perectly acceptable at f8 at the enlargement sizes I used. You can see this on teh easel.
Hope this helps. It is simple, cheap and so easy/obvious someone will certainly say, "Tom has nicked this idea from some book or other." However, I have not heard of this anywhere. It certainly beats using spirit levels on neg cariers, head, lens etc as this is rarely accuarate enough and too hard to see small variations...but is a good startin point. Most grain magnifiers will not get to the corners of a neg precluding using grain as a reference, so it means you can check across a neg area by eye.
Hope this helps,