Dont got a screenshot or video ready but I recall 2 sources.
1: Gale has a scene in camp where he asks you what you know about ceremorphosis. And then goes into detail about the grueling process. And how we are barely if at all progressing through the process. IIRC it triggers after you get your first step into the process or somesuch. Which is an indication that it is still happening, but very slowly.
2: The Mindflayer in the underdark offers to help you but comes to the conclusion that he cant because the tadpole is shielded or put into statis or somesuch. Hes never seen it before and cant help you because of it. But he also cant extract the tadpole without killing you.
There might have been more small sources but I cant recall them at the top of my hat.
