the same reasoning, they'd use to determine lenny the Leprechaun's, francis the Fairies, or elsa the Elves, existence.
Proving Existence or Non-Existence.
The existence of a thing can be conclusively proved by producing one single instance of the thing.
To put that another way: -
When the existence of a thing is denied, This can be proven wrong by producing one single instance of the thing said not to exist
The non-existence of a thing can never be conclusively proved because there is always the theoretical assumption that the thing exists but has not been seen yet or it exists in a place that can not be visited. Unless all places in the universe have been visited and are being constantly observed, we can not be absolutely certain.
From this we can say that there are only two possible statements we can make about the existence of a thing:
The thing exists.
It is unknown if the thing exists or not.
It is not possible to prove that a thing "does not exist" without further qualifying criteria.
If a thing does NOT exist it can not leave any evidence of it's non-existence. Only things that DO exist can leave evidence. From this we can derive that conclusive proof can only come from the person that claims that a thing exists. It is nonsensical to demand proof of non-existence.