There isn’t a reliable quick check mentioned here for knowing exactly when D-76 is exhausted. A cautious approach is to follow Kodak’s published capacity and storage guidelines and be conservative about both shelf life and the total amount of film you run through it. If you want extra reassurance, you could test with a small strip of film, but the safest practice is still to retire the developer before you reach the limit rather than risk important negatives. In general, developer is cheaper and more replaceable than exposed film, so it’s wise to discard it a bit early rather than push it too far.