I was serious, though, and not series. My beeber is not stuned.
How does one know that the artwork with phallic adornment was part of the piece as originally designed and executed?
One does not know that for a certainty, but perhaps one can know that "beyond a reasonable doubt." For one thing, the elements of the piece — the dead man and the bull — appear to be strongly correlated: The artist gives us the bull in full "murderous" charge, which entails the lowered head and the tail, which is rigidly extended outwards from the bull's body, which is characteristic of charging behavior.
In the second place, by now practically every square inch of this large suite of caves has been gone over in excruciating detail by several generations of scientists. The pigments used have been identified and analyzed; and all are seemingly contemporaneous.
If graffiti were involved, it would have had to take place before the discovery of the cave complex in the 1940s. Since that time, the French Government, via its Ministry of Culture, has done a superlative job of protecting, maintaining, and preserving the site. It is no longer open to the public (because of carbon dioxide-caused degradation of the paintings occasioned by vistors' breath). Instead a facsimile has been constructed: visitors go there.
Plus the pigment dating seems to preclude a graffiti scenario prior to the 1940s.
There's nothing we can know in life with absolute certainty, other than we are some day going to die. Oh, and also that the government will tax us while we live. :^) If we had to wait for certainty before we could do anything, or draw reasonable conclusions, then we would not be able to do much at all, and there would be little if anything to reason about.
Thanks for writing, grey-whiskers!