This can be done by using the profiler in the tests, as the mailer profiler tracks mails: http://symfony.com/doc/current/cookbook/testing/profiling.html

@stof I know that this can be done I'm just emphasize the fact that there is no cookbook on how to check that a mail was send.

BTW it seems that the correct profiler is named swiftmailer (and not mailer)

@weaverryan should get a Doc request label

I can take this one, is there still interest on it?

The thing is that the cookbook might be really short...
There is not much to say actually as it is pretty straight forward to check mails in profiler

@ricardclau, yes, it would be great if you document this! I don't think we shouldn't document things because the article will be to short. It usually begins short but ends as great cookbook article (thanks to the active symfony community).