Good question.
I'll definitely answer this reversed question, once you answer OP question.
OK. Well it would have to be something rather different than what has eluded me so far - the words of others, the various writings, the claims of experiences leading to such, the apparent logic, what we see around us, life itself - all such could well be explained by suitable processes and time, and I'm suitably informed so as not to trust my own experiences, including any of my thoughts or feelings. So I'm not sure what might do the trick other than discovering some DNA sequence saying 'made by God' (like a stick of Blackpool rock), or some event that would put the question beyond doubt, and that could not be explained in any other way. But I doubt this is going to happen.