RuntimePlatform.OSXPlayer  

if (Application.platform == RuntimePlatform.OSXPlayer)
    Debug.Log("Do something special here!");

using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
    void Example() {
        if (Application.platform == RuntimePlatform.OSXPlayer)
            Debug.Log("Do something special here!");
        
    }
}

import UnityEngine
import System.Collections

class example(MonoBehaviour):

    def Example():
        if Application.platform == RuntimePlatform.OSXPlayer:
            Debug.Log('Do something special here!')