Pidge,
The word God in every language. What is lacking for it not to count as evidence?
Some method of verifying the claim. Unless this god was interested in persuading only the gullible and the hard of thinking, he'd have to do a lot better than taking out a billboard with his name on it. If ever such a phenomenon was observed though, it would tell us only the following:
1. The observed event was not congruent with our current understanding of the physical universe.
2. Unless we found some means of understanding every possible aspect of the physical universe, we could not eliminate the possibility of the event happening naturally.
3. Even if we could with certainty understand every possible aspect of the physical universe, that would tell us precisely nothing about the cause of the event. Presumably anything capable of causing it would also be capable or producing whatever word it liked, regardless of whether it was a god or something else.
This really is basic stuff you know, even for you.