The fz6 is a very reliable, trouble-free bike. If I were looking at used bike from someone I didn't know, I'd check for obvious signs of it having been dropped or crashed -- scrapes and bent parts.
Is the rear tire bald down the middle? If so, could be that the rider was just a massive commuter or it could point to the rider being a squid out doing burn-outs.
Parts that may be marginally in need of replacement could likely be considered "consumables" -- things such as brake pads and the chain. You can see the brake pads on the front brake easily enough. Run a finger over each of the brake rotors, are there any serious grooves or scratches? When you ride the bike and apply the brake, do you feel any weird feedback through the lever? If you do, it could suggest a warped or bent rotor.
Ask when was the last time she/he flushed the brakes. If they say "never", well, this will be something you'll want to plan to do. If they haven't done it, it's not that they're absolutely guilty of poor maintenance, but some here would beg to differ on that one.
Check the chain and sprocket condition and tension -- too tight or too loose may suggest less-than-the-best care.
Squeeze the front brake and try and rock the bike front to back. You're feeling for a slight clunk in the steering bearings. The fz6 doesn't get the best steering bearing set and, if the owner hasn't already replaced it, you may want to just consider doing the job once you take possession. A very nice replacement set can be had for about $30 and the install job is not that difficult.
Ask about oil changes. Frequency of changes and what kind of oil. If they're not using [insert flame-worthy brand and weight here], walk away, just walk away...
