Was there ever a case where one has to prove access to materials cited?
I could interpret your question two different ways: (a) proving that you have read the article (because otherwise your research is academically dubious), or (b) proving that you were legally entitled to read the article (because otherwise your research is legally dubious).

If the material is published, then you do not need a license to read it, you only need a license if you want to copy it. It is perfectly legal to read it by looking over someone's shoulder on the bus or in the library. Therefore, you do not need to prove that you were legally entitled to read it.

On the other hand, if you don't have access to a copy that you can go back to any time, you are going to have trouble responding to any queries from referees that suggest you didn't read it carefully enough...

As for unpublished material, such as private letters, I think it would be prudent to refrain from citing the material unless you have legitimate access to it.