When I'm reviewing items I sometimes come across answers like this:
Here's what you are looking for: www.domain.com/resource.aspx
How should one handle these? By adding a comment?
Yes. In fact, in the Low Quality Review Queue, there is a canned comment for exactly that kind of answer:
While this link may answer the question, it is better to include the essential parts of the answer here and provide the link for reference. Link-only answers can become invalid if the linked page changes.
You could use that, even if you encounter the answer outside of the Low Quality Review Queue. Or, you could use your own wording to politely explain to the user that Stack Overflow answers should stand on their own, and that link-only answers are susceptible to link-rot.