This should actually be quicker and easier to prove than something like gold ETFs (e.g. SPDR) or other commodity ETFs.
The custody provider (e.g. Coinbase is the custody provider for most of these ETFs) should be able to simply show wallet addresses for all the Bitcoin they hold.
The custody provider (e.g. Coinbase is the custody provider for most of these ETFs) should be able to simply show wallet addresses for all the Bitcoin they hold.