Application.platform

Description

Returns the platform the game is running on (Read Only).

Use this property if you need to do some platform dependent work.

using UnityEngine;

public class ExampleClass : MonoBehaviour
{
    void Start()
    {
        if (Application.platform == RuntimePlatform.WindowsPlayer)
            Debug.Log("Do something special here");
    }
}

Additional resources: RuntimePlatform enum, SystemInfo class.