You are right about the tailstock as a primary reference it is inadequate. The first job would be to get the laser to point at a target at the tailstock end. When the dot doesn't move with the lathe turning (by hand) then the beam is directly on the axis. Then you can set up the tailstock centre, before checking the carriage. if you only want a relatively accurate level how about this one http://www.chronos.ltd.uk/cgi-bin/sh...html#a30316004
You can always put it on the bed, zero it and turn it 180deg to check for any slope.