I'm not an expert in such things, perhaps other, more mechanically knowledgable riders could chime in, but...
Smooth operation, instant start up, oil color (should be dark golden, not black), usual stuff. The engine should have a very nice "whirl" to it when you take off. With the transmission noise, its been described as almost like a low rpm jet engine. At highway speed, you should not hear the engine at all, almost, except when accelerating.
When decelerating the transmission gears will have a similar, higher pitched whirl.
In "Power" mode, acceleration should be crisp as well as have a very pronounced engine deceleration. You should have no troubles surpassing any vehicle in this mode. 0-60 mph in 7 seconds I believe.
In Regular "drive" mode the ride should be a good deal more pleasant, and acceleration and deceleration not so pronounced. More of a Sunday drive feel to it. The clutch will stay engaged until 10mph when slowing, then you'll feel the clutch release and a nice coasting affect.
The brakes should be equally powerful. Try a hard stop, when safe. You should feel very confident in its stopping distance.
The speedometer is a reported 8-10% high. There should be no sputters, or jitters or popping sounds, just smooth, controlled operations, like a well healed luxury sports sedan.
Now, I've only had my 650 for a few months and only have a few thousand on the clock, but those are my impressions. Perhaps a high mileage rider can give more insight.
You are buying, arguably, the finest scooter on the market. There should be no doubts as to this when you ride it. This bike is smooth as silk. If something feels or sounds wrong, there probably is.