If I look at
I see all the users, but some have given a bogus email and never used their password.
Is there some way to see who has used their password at least once? If not, is there a plugin to do this going forward?
If you install the following plugin and leave that one running for a while... you can see who logged in since using that plugin:
That seems to do what you need.
As far as I quickly can see it runs on $this->hook( 'wp_login' ) and does:
in the meta table can help somewhat on determining the history before installing the plugin, never used it (see http://wpseek.com/wp_user_settings/)
Login information like this is not stored anywhere in database. But you can add it for future users. Following code will check for current user meta
Following code adds additional column