I just constructed my stock D76 developer and am beginning to use it.
How can I check when it cannot be used anymore for further development?
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I don't know of a way of knowing if it is exhausted (you could process small test lengths of film perhaps). Rather, I would follow Kodak's guidelines (PDF link) for it, probably being rather conservative about both how long it keeps and how much film to process: film is much more expensive than dev, and exposed film is irreplaceable, so wasting a little dev by under-using it is a lot safer than possibly ruining film.