Kraken is getting the shout out for being the first exchange to take some serious steps towards proving their bitcoin solvency. Their solution ( https://www.kraken.com/security/audit ) is not perfect but it's better than what any other exchange has done so far. It's going to be just 2 weeks because it's not a full proof of solvency i.e. it still depends on trust to the auditor. I'm happy to promote them because based on my talk with them my subjective opinion is that they know what they are doing.

They plan to release audits quarterly. I definitely would like to see these more often, and with direct proof of control of the bitcoins. I hope this is just a first step to a better future when such proofs become a standard. But a lot of this depends on the community. You can ask your exchange to improve their transparency, and you can vote with your money.

Relevant links:
https://iwilcox.me.uk/2014/proving-bitcoin-reserves
https://github.com/olalonde/proof-of-solvency