Description

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");
        }
    }
}