Try to search for a GUIStyle. This functions returns NULL and does not give an error.
using UnityEngine;
public class Example : MonoBehaviour { // Checks if a style name exists
string aStyleName = "A Style I have";
void OnGUI() { if (GUI.skin.FindStyle(aStyleName) == null) { Debug.LogWarning("No style named \"" + aStyleName + "\" could be found"); } } }