Hunting down a good lathe of any make is, of course, a problem, and for me to give you specific advice about condition is difficult. However, in general, and briefly:
- check the bed for damage or wear steps near the headstock - or "cut-in" lines where the saddle has worn it. Some slight damage is inevitable - but chunks torn out are not. Although chips and small marks may not affect the accuracy they do indicate careless use
- find the nut that locks the saddle to the bed. Tighten it just sufficiently to let the carriage move near the headstock. Now move the saddle towards the tailstock. If it slides all the way the bed-to-saddle fit is excellent - if it jambs after 300 mm (12 inches) or so, the bed is badly worn
- trying to lift the saddle off the bed will tell you nothing. Some lathes have no keeper plated between the underside the saddle and the bed (and actually there is no need for them) and lift will not indicate wear
- lift the control levers on the apron and headstock up and down to check for shaft play - a good indication of general wear and difficult to disguise without lots of dismantling
- engage the leadscrew clasp nuts and check to see if the saddle can be pushed backwards and forwards. If it can, showing wear in the clasp nuts, the lathe will have been working hard
- hear the machine run on every speed
- run on top speed for a least 15 minutes to check for an increase in noise or rumblings - easily disguised by colder and thicker oil
- check the "backgears" on the headstock if visible (don't forget to rotate, fully, both spindle and backgear shaft). On lathes like the Boxford where the gears are hidden away inside, use a small torch to inspect)
- engage the backgears and check for noise. They are never silent but should run without rumbling or vibration.
- try EVERY position of the feeds and screwcutting gearbox (if fitted) and check that each works
- extend the tailstock spindle for 60% of its travel and check for up and down and in and out play
- check the cross-feed screw for backlash. This is one of the most used part of the lathe and it will be surprising if some play is not evident. However, it's common for the screw and nut to be replaced several times during a lathe's lifetime
- if you can, turn a test piece - and take a second pairs of eyes with you - it's surprising what an independent observer can pick up.
- ignore the paint finish, it can hide a multitude of sins. You are after a work tool, not an ornament.
- like buying a car, don't be in rush, there are lots out there and a good one will come along eventually.