This meta.SO answerThis meta.SO answer covers the steps pretty well but I will sum it up here.
First and foremost, be as kind as possible. As with anything, assume good faith.
Check to make sure there isn't a source listed somewhere that you might have missed. If there isn't, edit one in if you can find where the text came from, which is usually easy enough to do with a Google search. Attribute, link and block-quote as necessary.
Leave a friendly comment alerting the OP. Be nice! :) Sometimes people just don't realize they need to attribute, or forget, or the like.
If you notice that one user seems to have a pattern of plagiarism, flag it for further moderator attention. In the flag, mention you suspect plagiarism, and leave sources for their unattributed quotes if you can, so the mods can follow up on it. As the site gets bigger and more popular, the more you can help the mods quickly find the things they need to make a decision on how to handle things, the better. Makes life easier for everyone if we all help each other out!