The procedure is not EXACTLY the same, but it's slightly similar.
I came across this question about if Muhammad Ali said "...impossible is nothing...".
I never managed to conclusively and definitely prove it wrong, but after looking for quite a while I never managed to find any proofs of him actually saying it. So I went from the other end of it, to try and prove adidas made it (in your case that would be who made it up about the cross).
I found circumstantial evidence of it. Like the phrase appearing on google around same time as Adidas started with the ad. Wikiquotes not listing it, not finding it in any books before the adidas ad etc.
Even if you can't prove something wrong, you can at least give out several pointers of it probably being wrong, because of several circumstances.
In the cross question, it's quite a bit harder though, since it's so many years ago. Tracking down a quote from a person from modern written time is quite a bit easier, and that took me days.
TLDR; Try to find circumstantial evidence that strengthen your point if you can't find direct evidence. And be happy with saying it's improbable that it's true.