One of the best things about Gmail is its robust filtering system. It's easy to route very specific types of messages to different folders or relegate them to the archive easily. Now, you can supercharge Gmail's default options with these system labels that let you search unsearchable folders or root out specific types of messages.
Lifehacker reader Daniel turned us on to these new labels, mentioned by our own Gina Trapani on a recent episode of This Week in Google. Alex Chitu of Google Operating System has a short list of some of the most interesting system labels you can type into the search bar (along with whatever else you're looking for) to search all folders, including spam, root out Google+ notifications, search only messages labelled "important", find only messages with unsubscribe options and so on:
^g: muted conversations (just like is:muted or label:muted or label:mute) ^p: messages marked as phishing ^op: messages automatically marked as phishing by Gmail ^os: messages automatically marked as spam by Gmail ^vm: Google Voice voicemail messages (just like is:voicemail or label:voicemail) ^io_im: important messages (just like is:important) ^unsub: messages that include unsubscribing options. Gmail offers to unsubscribe on your behalf ^cff: messages from your Google+ circles (just like has:circle) ^p_esnotif: Google+ notifications For example, you can find messages you've marked as spam by searching for [l:^s -l:^os] or [label:spam -l:^os]. From all the spam messages you exclude the messages automatically marked as spam by Gmail.
This is just the tip of the iceberg. Mihai Parparita or persistent.info essentially tried dozens of potential filters manually to see what happened, then documented the results. You can see the full list of system labels and search operators at the links below.
Finding Messages Explicitly Marked as Spam in Gmail [Persistent.info via Google Operating System]