Easiest way to check for signs of bypass is to get under the front of the truck and look at the bottom of the radiator (side facing the engine) and look the connections for the transmission fluid cooler. If there aren't any hoses on these connections or the same hose is connected to both sides then the radiator cooler is bypassed. To check the transmission fluid you will need a 10 mm socked to remove the dipstick.
Other Pathfinder issues to look for:
While idling or driving slowly, rev the engine (2k or so) and listen for a whining/roaring noise. The secondary timing chain tensioner shoes are known to wear out and cause a whining noise. Being out of warranty this is a $1000-1500 repair at the dealer.
Then of course look for the general things (oil leaks, make sure all guages are working, no check engine lights, uneven tire wear, smooth solid braking, signs of being wrecked or major body repairs, rust)