How can I find out if an e-wallet service uses a fractional reserve system? A comment on this answer pointed out that the total amount of bitcoins in circulation could appear to go above the limit of 21m if wallet services use a fractional reserve system. However, in that scenario you actually no longer have bitcoins - instead, you have some kind of bitcoin-credits that rely on a specific 3rd party to be used.

If my understanding is correct, it should be possible to determine through the blockchain whether a given e-wallet service:


Definitely isn't using fractional reserves for a given account, or
Might be using fractional reserves


How would this be done? Has anyone summarised this information for a selection of e-wallet services?
There are two types of EWallets, hosted (shared) EWallets and hybrid EWallets.  

With hosted EWallets, all the coins are shared (all customers funds are combined) in the host's EWallet.  Examples of this are Paytunia.com, Instawallet.org, EasyWallet.org, WalletBit.com,  as well as the online wallets with the exchanges, such as MtGox.com, Bitfloor, etc.

Hybrid EWallets are those that operate locally either from the browser (using Javascript) such as how Blockchain.info/wallet and StrongCoin.com operate or from a client like how Electrum (Windows, Mac, Linux, Android) or BitcoinSpinner (Android) operate.

With a hybrid EWallet, the private keys are kept locally but transactions route through the EWallet provider.  To protect from loss, the browser-based hybrid wallets store an encrypted copy of the data from the wallet online with the EWallet provider.  The provider doesn't have the password to decrypt the wallet stored with them and thus has no potential to spend your coins stored with them.

So your risk of fractional reserve exists only with the hosted EWallet providers.

One method these providers could prove they are not a fractional reserve would be to submit to sporadic financial audits.  None do this today.

Another option would be for the EWallet provider to treat each account as if it were a standalone wallet.  That way the blockchain would verify that the coins received have not been spent.  No EWallets operate this way either.

When multisignature becomes functional, this will likely be a security feature available from one or more EWallet providers.