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They are usually marked, but sometimes you can look underneath or at the valves and tell. The larger one is 3" and is the black tank. That is correct about the tanks. Only the black tank has toilet material. If you can attach a photo to your post of that area I may be able to offer more here. All follow ups are included in our fee.
Just checking back in to see if you were able to resolve this issue or do you still need more input from me here?