How do I determine which is spark plug #6 2002 Nissan Maxima in order to change the coil in just that cylinder?

From the front of the engine where the Belts are. The nearest bank of cylinders are numbered 2-4-6 so the one you are looking for is furthest to the right. If you have a scan tool you could plug out one of the coils to see which shows up a fault code. Hope this helps. Regards Jim...

What engine do you have in your Nissan that has the misfire, how many cylinders and how many liters???

This model uses direct ignition, which means each cylinder has a coil sitting on top of the spark plug, no distributor.
So - - air filter - fuel filter - spark plugs.
There are things that need to be done at a certain mileage which would be in your owners manual. Such as the serpentine belt, trans fluid change, coolant fluid change, etc.

most likley a ignition coil is failing, is unfortunatly common for the A33 maxima. these engine have 6 coils, one for each cylinder. the only way to work out which of the 6 is the faulty one is to remove the little cover over the front bank of spark plugs, has 4 allen key bolts, then you can see the front 3 coils and the rear 3 are inbetween the manifold pipes. each coil wiring will have a button/leaver on the connectors (i think they are green) and if you push this in will release connector.
when the engine is missfireing unplug one coil at a time then reconnect after. if the engine runs worse that coil is working. if you unplug 1 that makes no change, that is the the one thats not working. replace the coil and the spark plug.
do not worrry, there is only 12volts at these connectors, not spark plug voltage

Spark plugs are located under the ignition coil for each cylinder. Each coil has to be removed to gain access to the spark plug for that cylinder.

The Spark Plugs are located under the Coil Pack in the Cylinder Head. You need to remove the silver cover on the front part of the engine which are held down by Allen Bolts to access the Coil Pack. You will need to remove the two bolts holding the Coil Pack to the Cylinder Head. Pull up on the Coil to remove the Coil and inside the hole will be the Spark Plug. The rear part of the engine next to the firewall has no cover so you will need to just remove the Coil Pack. The Nissan Maxima is very picky about the brand of Spark Plugs you replace them with, You should only use NGK Platinum Plugs or Better in your Maxima, Any other brand will make your car run rough.

spark plugs are screwed into the cylinder head and have spark plug wires leading to them from coil packs or the distributor.

For the 3 spark plugs in front:
1. You would have to remove the plastic protective cover (this cover is silver&red w/ the words "nissan and v6 3000")
2. unplug connector and remove coil (DO NOT mix up the coils, keep the same coils on the same cylinder)
3. Remove spark plug

Rear 3 plugs: located on the rear of the intake (the black pipes atop the engine). there are 3 openings at the rear of the black intake pipes, each opening has a coil mounted.
1. Unplug coil connector and remove coil
2. remove spark plug (you'll need a long extension)
***Installation is reverse, be sure not to over tighten the plugs!! Use anti-seize compound on all the new plugs threading. Hopefully this helps!!

There are 6 coils that fire the spark plugs. You only need to replace the coils that are misfiring. Some autoparts stores will let you use a diagnostic meter for free. The code will tell you which cylinder is misfiring and the autoparts store can tell you the firing order so you can locate which cylinder & coil.
Unless your car is due for a tune up I would just replace the faulty coil and not the spark plugs. Dealer will always recommend replacing all the coils. Eventually another coil may fail but who knows when that will be. When you notice the symptom again just tackle it as you did this one.

This car has seporate coil packs for each cylinder, have you tried swapping the front bad one with a good one to see if they switch ?
The engine you just put in may have come with some blown coils.
Hope this helps
Sam