Welcome aboard the board Qwazert.
Ask the seller to provide the maintenance history documentation, this is your best source of information. I look for regular oil and oil filter changes. Depending on the mileage, the first drive belt change should have occurred at 25,600 km. There are also other maintenance between 19,200 and 25,000 km like brake fluid change, spark plug and air cleaner replacement that should have occurred If seller does not have this, you'll need to do a thorough inspection (and likely perform these maintenance routines at your expense).
I always ask that the seller NOT start the machine (I check the muffler to ensure it is cold) prior to my arrival so I can check things like cranking strength, any smoke or unusual smells coming from tailpipe on startup and idling smoothness. I look for drips, leaks, accumulation of grease etc. indicating something is leaking or weeping. I pull the dipstick, smell it (you don't want a burnt smell, and check it for colour. If dirty, thick black, indicates lack of regular maintenance.
Second, visual inspection of tire tread, wear pattern and DOT date stamp, brake pad material remaining and amount of corrosion around the disc and caliper parts.
Look for signs of falls at handle bar weight ends (scrapes), obvious scratches to the body panels, look deep under the two side rocker panels ... usually they'll tell a story if the bike's been down.
I start it up and watch how even the back wheel rotates. Obviously a test ride is ideal to check wheel balance, steering integrity, braking power, functioning of the drive train and suspension dampening performance.
Cosmetics ... well that's usually pretty self-evident.
In Ontario, seller's must provide a UVIP (Used Vehicle Information Package) which show existing liens, vehicle ownership history, any branding (i.e. salvage, rebuilt etc.). Maybe run a CarProof if a UVIP equivalent is not available in BC.
I'm sure other members will post up with their their buying inspection tips.
Good Luck and keep us posted as to what happens.