I would do an cold start and listen very carefully for rattles, tapping, etc... coming from the engine when it's cold. Same check once it warms up. Check the exhaust temps on each pipe to see if they are all about even as it's warming up. A cold pipe is an indication of a problem.
I would fully inspect the chain as it's a clear indicator of abuse / neglect.
Some of the more expensive fixes are in the valves, timing chain, head work.
Generally the 919 is a rock solid, bullet proof bike, so I'd focus on the engine and any odd sounds. Maybe hold the rpm steady at different spots and listen for anything odd.
Depending on the price, you might want to look close at the fork seals, chain/sprocket condition, etc... if it's cheap enough, these might be know to need replacing and the price reflects that. IMO, the 919 is really a bargain compared to the Monster, ZRX, and others, as I've seen pretty nice < $4K 919s, and that's a pretty good deal IMO.
This guy bought one and it has a pretty good problem that IIRC, was pretty expensive to fix: