You need to look at the [search documentation][1]

>Boolean Operators<br>
>The following search operators can be used with values of yes/no, true/false, or 1/0 (each pair behaves the same):<br>
>...<br>
>wiki: yes/true/1 returns only community wiki posts; no/false/0 returns only non-community wiki posts.


So something like <a href="http://serverfault.com/search?q=is%3Aquestion+wiki%3A1">`is:question wiki:1`</a>
  [1]: http://serverfault.com/help/searching

Would find all Community Wiki questions. 

Similarly <a href="http://serverfault.com/search?q=is%3Aanswer+wiki%3A1">`is:answer wiki:1`</a> will return only answers that are Community Wiki 

and 

<a href="http://serverfault.com/search?q=wiki%3A1">`wiki:1`</a> will return anything that is Community Wiki.