Short version: you can count reserves, but nobody outside the exchange can independently count liabilities — that asymmetry is why "paper bitcoin" can't be strictly ruled out.

What works: reserve attestations. The exchange publishes the addresses it controls and signs a message from them at a snapshot, so anyone can sum those UTXOs on-chain themselves. Major exchanges run these monthly now (OKX just did its 45th round in July 2026), with wallet assets vs account assets broken out per asset.

What doesn't fully work:

1. Reserves aren't solvency. A Merkle-tree proof-of-reserve lets each customer verify their own balance was included in the liability total, but the liability side is still self-reported. Holding $5B against $6B owed passes every reserve check there is.
2. Rehypothecation. One UTXO can quietly back claims at multiple custodians or lending desks. The chain sees 1 BTC; claims may exist for several.
3. Snapshots are gameable — borrow before the snapshot, return after.

zk-STARK-based proofs have been tightening this up, but they prove inclusion in what was reported, not that the report is honest. So: guarantee? No. Best achievable today is signed control of addresses + Merkle-tree liabilities + third-party audit all taken at the same instant — and even then only for the custodian who cooperates. Self-custody remains the only supply claim you can verify end to end.