Auto-collimator or laser Interferomter are options, but it all depends just how accurate you want to go.

No point going for guides in the accuracy of tenths over their length, when the spindle flexes a couple thou under any kind of load.
Also, unless you get the base heat-treated/stress relieved after welding, it'll gradually flex as the weld induced stresses gradually relief themselves over the next few years, throwing your original accuracy out.