概要

using UnityEditor;
using UnityEngine;

public static class Example
{
    [MenuItem( "Tools/Hoge" )]
    private static void Hoge()
    {
        var gameObject = Selection.activeGameObject;
        var isChild    = !PrefabUtility.IsAnyPrefabInstanceRoot( gameObject );

        Debug.Log( isChild );
    }
}