What evidence is there that the historical individual that I'm thinking of fits a historical theory that I read one time.?
If the question relies on information that is only available to the OP, then the question shows no research effort, and I would argue is a bad fit for the site. I'm going to downvote because it is an insult to the community. Questions like this one are very likely to be consider for closure as "I'm not sure what you're asking."
If you must ask such a question.....
The great tragedy is that these questions are, in my opinion, simple to transform into valuable questions. Provide your source. If you're asking about a historical figure, provide a link to that historical figure. Not only do you vastly increase the value of the site as a reference source and learning aid, but you may disambiguate the question. There are many historical figures who are known by different names, and many that share the same name. If you're wondering about some historical theory, then provide a link to that theory. If you provide me a link to an interesting historical theory that I've never heard of, then I'll upvote your question, praise you publicly and instruct the network of telepathic goldfish to send good vibrations your way. But more importantly, if you include the relevant details in your question, you increase your chances of getting a good answer.
Example
Jesus in IndiaJesus in India - My objection is not about religion; my object is that the question asks for evidence about a theory that (as far as we know) is only in OP's mind. "What evidence is there that Jesus visited Schenetady N.Y.? What evidence is there that Mithradates visited London?"
@Vector did some research and found what plausibly was the source that OP was thinking of, and I think that @Vector deserves multiple upvotes for rescuing the question and for research an answer. But since OP chose to ask the question as a guessing game, we don't know if that is the source, or if the OP received telepathic messages from a nearby goldfish.
Counterexample
This questionThis question (singer and applause) is different; @Voitcus admits that memory is the issue, and even provides some details of the research that he's done. There is even a hypothesis that we're invited (implicitly) to help prove/disprove.
Or look at The Eastern QuestionThe Eastern Question; @JFW takes the time to offer definitions of the terms used in the question and the context. I learned something just reading the question, which is kind of cool.