Oil leaks under the clutch bell housing and clutch slip when throttle opened hard in high gear (Main output shaft oil seal). A lot of DIY work or expensive at dealers.
Oil leaks around the timing cover (not so expensive but a nuisance).
Check coolant level is correct and clean (no oil)
Idle the bike on the stand for 5 minutes or bring it back from a ride and let it idle. Check BOTH front fans come on at about 2/3 up the temp gauge (hot air from both side vents).
Any squirminess from rear rolling into and out of corners (swingarm bushes worn). Fixable but can be time consuming.
Check the rotors for any bad wear grooves (rotors are expensive). Also spin the wheels and see if they run true.
Make sure the ABS lights are both on at start up (NOT disconnected) and go off as you ride away. (ABS faults can be expensive to fix)
Any yellow gas stains on right side engine cover (indication of a fuel leak or failed quick disconnects) Fairly easy fix with metal QD's.
Bike on center stand, Wobble steering fast back and forth - should be some resistance from the steering damper - if none and steering feels loose, damper/wobble stopper is dead. Good boneyard or non-oem replacement, but many ride without it.
Most other things you probably know about, no used bike is perfect and others could throw in a lot more. I'd only look at things that might be expensive to put right and worry less about minor cosmetics.
If the bike has side rails and bags that will save you a lot putting them on later.
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