AudioClip.channels  

// Prints how many channels the attached audio source has
// 1 --> Mono
// 2+ Stereo
Debug.Log(audio.clip.channels);

using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
void Awake() {
Debug.Log(audio.clip.channels);
}
}

import UnityEngine
import System.Collections

class example(MonoBehaviour):

def Awake():
Debug.Log(audio.clip.channels)