They would have to publish their wallet addresses (and then anyone can see how much is in the wallets), and then they could send small transaction amounts to a trusted party from each wallet to prove they had control of the wallets.
So let's call this method AT("Appointed Transaction").So what if I have a lot of bitcoin, and I provide some service which are virtually illegal and immoral like this:
"I have 100000 bitcoins. If you wants to prove to other people you have such amout of coins, I will help you with AT auditing process. The price is 2% of the amout you want to be apparently under your control. My contact information is
LiarHeaven@XXXX.XXX.