To tell if what comes before the colon is a namespace or not, note the link for the discussion page for that particular article. If it is not a namespace, the talk page will begin immediately with Talk:; if it is in fact a namespace, the talk page will begin with that namespace, followed by talk:.
For example, the discussion page of this page is WoWWiki talk:Namespace, and discussion pages regarding users begin with User talk:. Note that the discussion page for Quest:Hungry! is [[Talk:Quest:Hungry!]].
Linking to articles in a namespace
If you don't want to have the namespace show as part of the type when you link to it you have two options to correct this. First is to type out the link as follows
[[:Category:Items|nice text here]] and get the results nice text here or you could simply type
[[:Category:Items|]] and get the results Items. Notice in the second option there is no text after the trailing pipe '
|' character. This is because MediaWiki will by default remove the namespace from display if it sees the trailing pipe and only show the name of the article in that name space.