I received a batch of old paper - different brands, surfaces, weights - and found varying degrees of fog. Supposedly all the paper was stored the same but I had no way of knowing the exact age of each package.

Some the the Agfa paper that I knew (by packaging) to be fairly old did surprisingly well. Others, in same style packaging, not so well.

Simple to test (essentially compare 2 partial sheets of unexposed paper; one that is developed and fixed and the other just fixed).