Answering a question for which you found the answer on another site doesn't mean to copy and paste what reported in that page. Links are used to backup your answer, such as when you give the link to the documentation page for a function, or a framework.
What you can do is:
- Showing an excerpt for that page with the most important points
- Making a summary of what the page says, using your own words
In the first case, the excerpt can be considered under fair use; in the second case you are using your own words, so there should not be any problems with that.
Apart from what said in the linked page:
- If the question is about code to write, and the linked page describes how that code should be written, you can add the code you would write basing on those suggestions
- Add your own comments about what said in that page, such as underlining what the common errors are, and pointing what the OP needs to carefully check
- Add your own expertise about the topic
How can you identify whether content is simply copied or "created", especially if the web page answers the question?
Having the link to the page, it is rather easy to see what part has been copied, and which part has been written by the author of the answer using her/his own words. The fact the linked page contains the answer for the question doesn't make more difficult to understand what part has been copied, and what part has been re-written using different words.