How should I act if I discover a user plagiarizing fellow users' posts? What if I find someone plagiarizing external resources without attribution?
Do not pile downvotes on recently uncovered plagiarists' answers, as this might trigger the serial voting script. Let the moderators handle it - they will destroy any occurrences of egregious plagiarism, and may suspend the user for a while.
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 legitimate source, and not one of the sites copying Stack Exchange content 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
From the Wikipedia Article on xyz: After that, proceed to step 3.
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
Plagiarism: copied without attribution from ....
If your flag gets declined, flag again and/or raise the issue on Meta. Here is previous discussion. Note that copyright is a separate issue from plagiarism.
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
Hi, it appears that you copied this answer from (insert URL here). You're using someone else's work without giving the author credit. This amounts to plagiarism, and is not welcome on Stack Exchange. Remember to **always** add prominent attribution when using other sources. Thanks!
or a more nicely worded suggestion from JMort:
Hi (User), it looks like you just copied most of this content from (this blog). Can you edit your post and give attribution to the author? Plagiarism isn't really welcomed on Stack Exchange, and it's always nice to give credit where credit is due. Good luck!
Or another option from wjandrea, more stern, but very informative:
It looks like you copied this from (URL). If you're using someone else's work without giving them credit, that constitutes plagiarism, which is not welcome on Stack Exchange. To fix it, you can , include a [link](/editing-help#links) to the source, mention the author's name, and [quote](/editing-help#simple-blockquotes) the copied content. For more details, see [referencing help](/help/referencing) and [this FAQ](https://meta.stackexchange.com/q/160077/343832).
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.