South Bend is a good lathe. Things to consider are: Is the headstock on bushings or bearings ? Bearings is better. Is the quick change gearbox fully functional ? Is the apron feed mechanism functional and does it engage/disengage smoothly ?
Does the cross slide move freely ? Is the cross feed functional ? Check for excessive backlash in cross slide screw ? Does the compound rest move ( I like it to be on the snug side)? Does compound rest swivel and lock in various positions easily ?
Is the threading half-nut functional and not worn out ( check carriage backlash when engaged ) ? Does it thread ? Does the bed have severe wear near the headstock ? Does the tailstock work smoothly ?
How well is the lathe equipped ? Does it have extra chucks, collet chuck w/collets ? Does it have a tool post ? Lantern style is ok, Aloris type is superior. Does it have steady or follower rests ? Accessories can cost as much or more as the lathe if you want a fully equipped machine
Now for the serious business... Chuck up a piece of stock about a foot long, put a dial indicator on chuck near you and push away then pull towards you. Any indicator movement ? A lot of movement here is not good. Reposition indicator above or below and check for play as done before. IMO, .001" or less is best, more then that will force you to learn the "secrets of the machinist trade"...
There is more but above should give you a good idea of what to look for. If you have any questions please PM me and I will try to help.