Brainwallet is now Closed Permanently[1,2] and thus there is currently no way to verify a signed message with having a client or using a service that requires a login. At least to my knowledge. If you know about a site that is able to verify signatures please let me know. For more about why the site is down see these[3] defcon slides or wait for the speech to be available as video. The tl;dr is: brainwallets are bad if done wrong and most do it wrong. My assumption currently is that this is the reason the site is down, which sadly also takes down the features that can be used without risk.

As Im currently not at home it will take some time till more verify guides are available.

[1] https://brainwallet.org/
[2] https://twitter.com/brainwallet/status/629759260004630528
[3] https://rya.nc/cracking_cryptocurrency_brainwallets.pdf