With the lathe....
I've got to admit I've never measured the alignment of the tailstock (mainly out of fear for what I would discover...) Mine's the old type (no camlock), and most people seem to start by remachining the dovetail on the base... Unfortunately I don't have a mill, so that's not an option.

I get away with it because my main use for it is hole drilling with jobber drills. The bores that I need to be accurate are usually large enough to true up with a boring bar (10mm dia+)

My only advice is before spending too much time on the tailstock, just make sure the headstock alignment is OK. Aligning the headstock is a nice excercise, and taught me alot about the lathe.

If you have an X2 then you should check out Daniel Kemp...aka Hoss2006 and see what can be done with it.
I think this is the site Kip is talking about.
http://www.hossmachine.info/