How should I act if I discover a user plagiarizing fellow users' answers? What if I find someone plagiarizing external resources without attribution?
Check again whether the OP didn't add a source somewhere after all. Sometimes it is at the end of the copied text, or hidden behind a single-word link. Check out all the links in the answer. Sometimes they get hidden by bad formatting - it's worth clicking 'edit' just for one final check to make sure.
Make sure that the original source you found really is a legit source, and not one of the ripoff sites copying Stack Overflow content (efreedom.us, etc.) or some other content aggregator.
2. Edit or flag
If it looks like a minor, single incident, editing the attribution in is the way to go. You can do that yourself, either directly or through a suggested edit. Put the attribution in front of the copied material, and put everything into blockquotes that isn't the OP's original speech. Use nice wording, like
If it looks like there is an egregious pattern of plagiarism, and deletions seem necessary, flag for moderator attention. Explain that you believe the answer has been plagiarized, and paste the source URL into a comment underneath, say
If your flag gets declined, flag again and/or raise the issue on Meta. Here is previous discussion.
3. Educate the OP
The OP may be innocent(-ish) simply because they're ignorant of how gravely wrong plagiarism is - this is the case more often than one would believe. If there is no indication the community has had a conversation with them about the issue, consider leaving a polite comment along the lines of
or a more nicely worded suggestion from JMort:
4. Go hunt for more
Moderators have enough to do already - they won't be able to check out a user's entire profile for further instances of plagiarism. If you feel like it, take that task upon yourself, and flag each occurrence as described above.