Lots of info if you search.
Other than typical stuff you'd look at on any bike, you'll specifically want to check:
- steering head bearings
- belt condition & idler bearing
- muffler rust
- engine case leaks
- low rpm hesitation & shifting action (esp on pre-2006)
- regulator/stator output & battery condition
- primary chain slack
- front brake rotor warping
Of course, when you are buying used, you are at the mercy of what's available, but to the extent you can choose between the 9 & 12, I think you'll be perfectly happy with a 9 which gives you the all the essence of the XB experience, usually for considerably less $ than a 12 of the same year and in comparable condition.