I went through this exercise, to align my scratch built VMCs linear rails, 2200 mm long.
I found a vertex ground CI straightedge, about 150€, new in the UK. Accurate to 0.04 mm.
A better option is a granite long parallel, but they are about 600 for 1.4 m long iirc.
Still thinking about it.
Machine builders square spirit levels, == 0.02 mm/ metre, were about 70£. I got two, and they make aligning stuff quite easy.
Testing the straightedge on a real granite surface plate, 600x440 mm, proves that you can easily see errors of about 0.01 mm, with a light behind it.
Putting a cig paper as a shim on either end, shows the lift clearly.
Since you dont need perfect, testing from the center to the edges should get you close.
Its feasible to sand steel pretty flat, ime.
I used a big, heavy, large belt sander.
Marking with a spirit permanent marker pen and sanding it off, shows you can get decent control on maybe 0.02 mm depth or so.
The edges get rounded.
The rails are mounted, and very well aligned, except at one spot. Its either high, low, twisted, or wanders from the front rail in some direction.
Plan is to put a dti on the ram, and check for rail error in the 3 axes.
Then just use a screw-jack to push it straight, and then tighten the mounting bolts.
You can easily feel any error over 0.01 mm while pushing the ram back and forth. Asain, cig paper as shims proves this quite easily..
The blocks are strong, and wont come to any harm from hand-forces.
I used 35 mm linear rails and blocks.
Hope this helps.
BTW - Pics of the machines will only come once they are polished, for commercial reasons.
Machine is commercial quality, with commercial components, and costs, and I need to clean them up before pics.
Hope to get them polished inside a month.. but this is not a reliable estimate.