Obviously check for signs of being laid down (especially handle bar ends because they can tell the tale)
If I was there I'd see if I could check the primary chain tension. I think that's a good indicator on him much love was given back to the bike. Also you get to check how clean or dirty the oil is :)
Also check for leaking engine covers after a test ride.
Sounds... Well on my bike all I can hear is an empty jardine and valves ticking!
I'd have to say if you've owned a few, you should know what to listen / look for.