Kraken regularly publishes proof of reserves. I don't recommend keeping funds on exchanges as a rule, but I don't feel stressed out leaving a percentage of my holdings there for an extended duration.
I believe only Coinbase publishes total liabilities at the moment. In my opinion, Coinbase is the gold standard due to being a publicly traded company and Kraken carries more risk.
>Any client can independently verify that their balance was included in the Proof of Reserves audit by comparing select pieces of data with the Merkle root.