Do keep in mind there is some variance in how the community feels about these, and this variance extends to moderators and Community Managers.
We all tend to agree that link-only answers are not ideal, but opinions on what to do about them vary from "leave a comment and let it be" to "nuke on sight".
What I say here briefly, then, will be from my own point of view;
Take a moment to consider the link, and its relevance. Does the link in question truly answer the question? If so, consider leaving the poster a comment (if it's from an active member) asking them to bring in some pertinent information (without plagiarizing) and include some of their own narrative about the answer.
If you see that's already been done and the active user has not reacted, you might consider flagging. "Not an Answer" is not often appropriate here, and low-quality is a bit risky. An "Other" flag explaining might meet with more success.
On the other hand, if the link in question doesn't at all directly address the specific question, you can consider flagging right away - and perhaps leaving a friendly comment, too. An "Other" flag is also useful here; "The link in this answer does not really address the question being asked" is a good place to start.
Keep these in mind when flagging, too:
- "Not an Answer" is meant for posts that are not an attempt to answer the question. A good, relevant link can be considered an attempt to answer. Answers (including links) which seem to have missed the mark of the question broadly don't generally count here, unless it's horribly egregious. But even then, an "Other" flag explaining it is a better bet.
- "Low Quality" means, "This post is a piece of trash that cannot possibly ever be improved and must be deleted - NOW. The posts we're talking about here can generally be improved, so this sort of flag is risky here.