Good question. There’s no button. Michael Potuck:
Metapho launched its latest update over the weekend that includes data on whether or not Deep Fusion was used in a photo (via MacStories). This is notable, as the feature was made by Apple to be intentionally invisible to users even when inspecting an image’s details (and it’s also very difficult to tell by just looking at photos, even side by side with the same conditions).
I tried it. It works. It won’t last long.