If privacy is not a concern, you can just publish a list of your liabilities at a given time. Each account holder can then verify the their own listing, calculate the total, and compare to the proven reserves.

There are more sophisticated schemes that allow you to check the amount in your account, and verify that your account was included in the total, without seeing the size of other accounts: https://iwilcox.me.uk/2014/proving-bitcoin-reserves .