How would you inconclusively prove that a reputation loan has taken place? You would need to know the intent of the borrower, and the last time I checked reading people's minds was not a science.
You cannot (usually) tell when a reputation loan has taken place. Sometimes a borrower will outright say that s/he is trying to build his reputation. Sometimes it will be clear that he did not actually need such a loan (he has much more money then was received and never touched such money while the loan was outstanding). Sometimes a borrower will quickly repay such a loan after receiving it. Sometimes the borrower will place the borrower in a situation that would make it impossible to not repay such a loan.