How do I know my eCash is fully backed by BTC?

Chaumian eCash is fundamentally un-auditable by design, and while you can see the total bitcoin assets / reserves of public federations on Fedimint Observer, this does not display federation liabilities – meaning the total eCash a federation has issued cannot be known.

Regardless, Fedimint Observer relies on the federations themselves for information on their reserves, which is why federations still ultimately require trust even if the trust model is unique. This is why you should always use federations in proportion to your level of trust in them.

If using public federations with anonymous guardians who you do not know personally, a simple stress test you can perform is to periodically rotate your entire balance out of one federation wallet and into another to ensure that transfers are being processed smoothly.

That said, keep in mind that transactions may fail for reasons other than malicious guardians, such as low liquidity on the federation's Lightning Gateway, poor connectivity with enough guardian servers to breach the consensus threshold, and other similar issues.