Checking for quaternion values to not be NaN

 if(!System.Single.IsNaN( someVariable ))
   // assign rotation

Well, i know this is a old post but, i will help others with simple soluction for check if "float variable" is a NaN or not.

 if(float.IsNaN("YOURFLOATVARIABLE")){
       //Your condition...
 }
 else{
       //Your "else" condition
 }

I have problems with either NaN or a zero quaternion showing up from time to time, and throwing an error somewhere inside Quaternion.Lerp.

To deal with that, I like to use an extension method - I keep a static class with a few of them in each project, utilities to do things like check if a UnityEngine.Object is null off of the main thread. Here's just the relevant part:

 public static class UnityExtensionMethods {
     /// <summary>
     /// Determines whether the quaternion is safe for interpolation or use with transform.rotation.
     /// </summary>
     /// <returns><c>false</c> if using the quaternion in Quaternion.Lerp() will result in an error (eg. NaN values or zero-length quaternion).</returns>
     /// <param name="quaternion">Quaternion.</param>
     public static bool IsValid(this Quaternion quaternion)
     {
         bool isNaN = float.IsNaN(quaternion.x + quaternion.y + quaternion.z + quaternion.w);
 
         bool isZero = quaternion.x == 0 && quaternion.y == 0 && quaternion.z == 0 && quaternion.w == 0;
 
         return !(isNaN || isZero);
     }
 }

This lets me write

 if(myQuaternion.IsValid())

before using a quaternion in any Lerps or assigning it as a rotation.

This worked on my browsers JS console:

 if (!NaN) console.debug("HI");

Hope this helps.