It's up to the author to specify. For most wikipedia text, you need to link back to the original article or a copy of the article or include a list of the major authors.
On the other hand, StackExchange is more onerous - they require a link to SX, a link to each author, each author's name, links to each author's profile, and a text blurb indicating that the content is from SO. Oh, and no "nofollow" links, or JS tricks - they want the google juice.
You also need to ensure that derivative work is also CC, if there is a -SA clause (which I think is the case for both wikipedia and SX.
It should all be there in the TOS page.
If they author/site doesn't say, just link back to them, and include a short blurb (i.e. "by <username> at <website>". Really, that's likely to be all they want. If they cared about the legalities, they probably include them in the TOS. Or email them, asking for clarification.
For Wikipedia, apparently someone did their homework: the disclaimer under the edit box says "You agree that a hyperlink or URL is sufficient attribution under the Creative Commons license."
SO's insistence on non-obfuscated links doesn't make much sense to me, though. Users retain copyright to the content they submit to SO, so it's them, not SO, who say which attribution they would like. SO could require some sort of attribution to be sufficient (like Wikipedia does), but users are free to say that they accept any sort of attribution besides that, even one which does not credit SO at all.
On the other hand, StackExchange is more onerous - they require a link to SX, a link to each author, each author's name, links to each author's profile, and a text blurb indicating that the content is from SO. Oh, and no "nofollow" links, or JS tricks - they want the google juice.
You also need to ensure that derivative work is also CC, if there is a -SA clause (which I think is the case for both wikipedia and SX.
It should all be there in the TOS page.
If they author/site doesn't say, just link back to them, and include a short blurb (i.e. "by <username> at <website>". Really, that's likely to be all they want. If they cared about the legalities, they probably include them in the TOS. Or email them, asking for clarification.