Input.anyKeyDown

Description

Returns true the first frame the user hits any key or mouse button. (Read Only)

You should be polling this variable from the Update function, since the state gets reset each frame. It will not return true until the user has released all keys / buttons and pressed any key / buttons again.

// Detects if any key has been pressed down.

function Update() {
    if(Input.anyKeyDown)
        Debug.Log("A key or mouse click has been detected");
}

using UnityEngine;
using System.Collections;

public class ExampleClass : MonoBehaviour {
    void Update() {
        if (Input.anyKeyDown)
            Debug.Log("A key or mouse click has been detected");
        
    }
}