Event.capsLock  

// Creates a Label and prints CapsLock on/off
// depending on the state of the capslock key.
function OnGUI() {
var e : Event = Event.current;
if (e.capsLock) {
GUI.Label (Rect (10, 10, 100, 20), "CapsLock on.");
} else {
GUI.Label (Rect (10, 10, 100, 20), "CapsLock off.");
}
}

using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
void OnGUI() {
Event e = Event.current;
if (e.capsLock)
GUI.Label(new Rect(10, 10, 100, 20), "CapsLock on.");
else
GUI.Label(new Rect(10, 10, 100, 20), "CapsLock off.");
}
}

import UnityEngine
import System.Collections

class example(MonoBehaviour):

def OnGUI():
e as Event = Event.current
if e.capsLock:
GUI.Label(Rect(10, 10, 100, 20), 'CapsLock on.')
else:
GUI.Label(Rect(10, 10, 100, 20), 'CapsLock off.')