You need to look at the [search documentation][1]
Boolean Operators
The following search operators can be used with values of yes/no, true/false, or 1/0 (each pair behaves the same):
...
wiki: yes/true/1 returns only community wiki posts; no/false/0 returns only non-community wiki posts.
So something like is:question wiki:1
[1]: http://serverfault.com/help/searching
Would find all Community Wiki questions.
Similarly is:answer wiki:1 will return only answers that are Community Wiki
and
wiki:1 will return anything that is Community Wiki.