If you go to a comment's page by clicking on the timestamp (e.g. https://news.ycombinator.com/item?id=15837946 https://news.ycombinator.com/item?id=15837946) yo

If you go to a comment's page by clicking on the timestamp (e.g. https://news.ycombinator.com/item?id=15837946 https://news.ycombinator.com/item?id=15837946) you should see a “flag” link.