What evidence would you need to see if he was trying to build trust verses some personal use?
You did a pretty nice list of criteria of reputation loans in your previous post:
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.
The biggest callout, however, (excluding explicit statements from the borrower that it was a reputation loan) would be if the loan money was untouched in the wallet.