check if a window resource exists without creating it

FindWindowInstanceExample.cs

using UnityEditor;
using UnityEngine;

public static class FindWindow
{
    [MenuItem("Debug/FindWindow")]
    public static bool DoesWindowExist()
    {
        var windowType = typeof(SceneView);        // substitute with your own type
        var allInstances = Resources.FindObjectsOfTypeAll(windowType);
        if (allInstances.Length > 0)
        {
            Debug.Log("found window of type: " + windowType);
            return true;
        }

        Debug.Log("did not find window of type: " + windowType);
        return false;
    }
}