How can i check the rear crank sensor on a 98 maxima

A crankshaft position sensor or a camshaft position sensor can both cause a no fire condition. Both of these sensors are magnetic sensors that communicate with each other and pass that information on to the computer, letting it know that the motor is in correct time. Of either of these sensors go out, the computer doesn't know if the motor is in time, and can keep the engine from firing.

If you just replaced the rear main seal, double check to make sure that both sensors got reconnected (if you had to disconnect them)first thing. After that, you might look into testing them. I'm sure you can do an ohms resistance test, but also make sure you're getting voltage from the connector while trying to crank the motor. Ican tell you from experience that those cars have issues with their camshaft position sensors. Good luck!

what are the codes that are coming up now?

seems like you got all you need , check the sparkplugs the engine may be flooding itself, (one guess),

did you replace the knock sensor from the old one to the replacement one. ? double check the sensors are the correct ones fitted to your original engine are on the new replacement, also check the connection plugs, sometimes these cannot connect fully if have become weak.

Is your check engine light on?

If the car is pinging you will set a knock sensor code.'
This comes from bad gas and running lean., You really need this professionally inspected to rule out the basics. I have seen alot of nissans with the knock sensor code stored but the light is still off.
Fill up the tank with high octane chevron and some techron injector cleaner and clear the light. If it stays off for a couple days then go get it inspected. Even if the code is stored it will still pass as long as the light is still off and the monitors have run.

Check crank sensors first.There are two sensors 1 is by the crankshaft pully (by the drive belts) the other is between the engine and transmission it reads from the flywheel . Good luck.

first check the condition of the spark plugs

The Crankshaft Position Sensor is located: Under hood, driver side, front engine area, mounted rear of crankshaft pulley
The Knock Sensor is located: Under hood, center, front engine area, below intake manifold, mounted on block

See this video:

http://www.youtube.com/watch?v=fPIMOIwLISc