A weird question. Based on this comment which got me wondering. If you were ignore all contemporary Roman sources, or consider them unreliable, could you prove that Julius Caeser existed?
Going by that comment, I'm just supposed to ignore written accounts of Caesar, which would still leave all the archaeological remains (eg. statues) of him. Leaving out all written accounts would of course make it difficult to use his name in the circumstances, but it would still prove his existence.
If we remove all known accounts of Caesar then? Well, obviously it's impossible to prove it then. The comment you linked to is quite ignorant on that part.