Try to search for a GUIStyle. This functions returns NULL and does not give an error.

	// Checks if a style name exists
	var aStyleName : String = "A Style I have";
	
	function OnGUI() {
		if(GUI.skin.FindStyle(aStyleName) == null) {
			Debug.LogWarning("No style named \"" + aStyleName + "\" could be found");
		}
	}

using UnityEngine;
using System.Collections;

public class Example : MonoBehaviour {
    public string aStyleName = "A Style I have";
    void OnGUI() {
        if (GUI.skin.FindStyle(aStyleName) == null)
            Debug.LogWarning("No style named \"" + aStyleName + "\" could be found");
        
    }
}

import UnityEngine
import System.Collections

public class Example(MonoBehaviour):

	public aStyleName as string = 'A Style I have'

	def OnGUI() as void:
		if GUI.skin.FindStyle(aStyleName) == null:
			Debug.LogWarning((('No style named "' + aStyleName) + '" could be found'))