I've assisted many gosling hatches and am currently incubating multiple batches of goose eggs.
In the context of assisting, when a gosling has internally pipped and you make a safety hole (mine tend to be on the larger side, so I can see the gosling's bill and ensure the membrane is clear of it), you should visually be able to tell if the veins are still active because they'll appear large and red in the membrane surrounding the gosling. If you moisten a bit of the membrane around the gosling with coconut oil, it will make it even easier to see the active veins because the membrane will become transparent. What you don't want to do is to damage the membrane and see blood, which is a clear sign that the veins are still active.
The key to assisting is to be extremely cautious, and go slowly. No hasty moves. Pyxis describes this in her article - and as someone who does this on a fairly frequent basis, I can't emphasize it enough.
