You can see special tags applied to the questions that indicate their status. These tags are rendered in a different color because they can only be added by moderators. Their names indicate what decisions was made regarding that particular bug.
The status tags include:
status-bydesign Indicates that a submitted issue is actually due to the existing design of the system and is not considered erroneous behaviour. In other words, "it's not a bug, it's a feature!"
status-completed Indicates that a feature request has been implemented, a bug has been fixed, or another type of request has been processed.
status-declined Indicates that a request (usually a feature request) has been considered, but will not be implemented.
status-deferred Indicates that the issue will be reviewed in the future.
status-norepro Indicates that a reported erroneous behavior cannot be reproduced by the development team.
status-planned Indicates that a feature request has been considered and received positively enough that its implementation has been placed in the development queue.
status-reproduced Indicates that the symptoms of a bug report have been reproduced/confirmed by the development team.
status-review Indicates that a feature request or possible solutions to a bug are set to be internally reviewed.
To see all of the bugs that have been marked completed, add that tag to the bug tag in your search query, like so.