It's hard to verify fiat for now. This could well be a feature of CBDCs that isn't practical with current deposits. Maybe you'll be able to look a the dollars on deposit with your EMI and follow the chain through their bank and up to the central bank.
I think the Tether conspiracies are exaggerated a little but also I wouldn't pick Tether as an example of best practice either. Enter some XAUt address such as 0x2faf487a4414fe77e2327f0bf4ae2a264a776ad2 into
Tether's website and you get a bunch of what look to be internal serial numbers. So not much use.
But if you go to
Paxos' website and enter a PAXG Ethereum address such as 0x28c6c06298d514db089934071355e5743bf21d60 then you can look up the gold︀ bar serial numbers at Gulidov Krasnoyarsk. That is something that presumably can be verified. Also︁ you could collect the serial numbers attached to all PAXG tokens and make sure they︂ add up. In future, I see the gold depositories having a similar lookup to (optionally)︃ allow the owner of each gold bar to be verified. Then you could really do︄ your own audit. No special magic needed, just publicly accessible, immutable data.
Beyond that, depositories︅ could tokenise as well and then you could follow the flow of trust from some︆ issuer you _really_ trust, down to your counterparty. Blockchain has famously been about Dogecoin and︇ NFT "artworks" going for millions but once there's been enough tokenisation of verifiable gold bars,︈ bottles of wine aging in the grower's cellar, stocks, apartments, offices, shopping malls, aircraft on︉ lease, etc., the use cases for smart contracts should explode. As well as critical mass,︊ this will require some maturity in terms of contract audits etc., which is where the︋ poly.network hack is useful.