By watching you pay at a convenience store (as an example). When somebody's wallet has a lot of cash in it, it shows.

$7.7 trillion is the amount of loan guarantees, not loans. Loan guarantees are still money in the government's coffers until they get defaulted on, and even then, there's no way ALL $7.7 trillion worth of guaranteed assets will default. If you count the loan guarantees as a giveaway, then the FDC is on the hook for a hell of a lot more than that.