Couple of things you could do. First, I would have the dealer give you a Carfax report on the vehicle, which would show if there was any radiator replacement or transmission work done to the vehicle in the past. Look at the top tank of the radiator. On Nissan radiators, the word "Nissan" is molded into the top tank and there should be a Calsonic sticker on the driver's side of the top tank, near the radiator cap. If you have a part number of 21400-EA215, -EA265, or -EA200, you have the original, factory radiator that is prone to failure. The good news is that if the ATF is not showing signs of contamination, which would be streaks of white in the red ATF or, in worse case scenario, a strawberry custard, chances are the radiator has not failed, but you do know you should have the radiator replaced or bypassed to prevent the possibility of cross-contamination due to cooler failure. If you have 21400-9CA2E, the radiator has been replaced, however, there have been a few failures reported with the early versions of these radiators, as well. If the radiator was replaced in 2010 or later with this part number, it should be okay. No cooler failures have been reported that I am aware of with the aftermarket replacements.
To check the dipstick, you'll need to removed the 10MM head screw that holds the dipstick, which is on the passenger side of the engine and has a black, rubber top.
As far as driving symptoms, the usual thing people have reported is a high speed "shudder" while driving. It's often the shift solenoids that get affected first, but the solenoids can't be replaced without purchasing the expensive TCM/valve body assembly. There have been a few that caught the problem early and despite a little cross-contamination, were able to save the transmission with several flushings of the fluid, but those tend to be the minority. If you do service the ATF, remember to not use regular Dexron III/Mercon. Nissan Type "J" or "S" ATF is specified by Nissan, which is the same as Castrol Transmax Multi-import ATF. Most aftermarket synthetics work just find and are less expensive, but make sure they state they are recommended for Nissan Type "J" and "S" applications. Personally, I use Valvoline Maxlife ATF in a 1-gallon jug, which works out perfectly for a drain and fill. The drain plug gasket is Nissan 11026-01M02.