I haven't had a Nomad in a lot of years, but here are some good things to look for:
1. Do all functions work on the amp? I wouldn't worry about the Slave out, but I would definitely make sure all channels work and change when they should.
2. Ask how old the tubes are. If the tubes are original or if the owner can't remember when they were replaced, you should count on purchasing a new set.
3. Ask if the amp has had any maintenance or repairs. This is not necessarily a bad thing.
4. Look in the back and see how dirty the cabinet is. Also look for holes in the speaker and missing reverb tank.
5. Run the amp through it's paces and try every knob in every channel. Scratchy pots are ok, but if they do not work or if the sound cuts out at a certain point, then you may have to negotiate a technician visit into the price.
6. Take a delay or EQ pedal with you and try the loop out. Don't use a distortion or compressor pedal, it won't sound very good. Take your own cords too. I passed up on an amp a long time ago then found out later that it was just a faulty cord.
7. Use the footswitch and make sure that it doesn't freeze up and that it changes all functions properly. If the footswitch does not appear to be working, make sure that the knob on the back is set to 'Footswitch.' If it is, then try changing the amp's channels without the footswitch. This will let you know if the problem is with the amp or the cord/footswitch.
8. Play it loud for at least 20 minutes. This will hopefully let you know if there are any heat issues.
9. Inspect the cabinet. Some dings are expected, but make sure that there are no cracks or separation at the seams.
Hope this helps!
Andy