The easiest way to check is to use curl on the command line. On Mac/linux Open up a terminal, on Windows open the Start menu, type cmd in the search box, and press Enter. Then type in:
curl -svo /dev/null https://www.yourdomain.com/your-resource
This will return the response headers for your request. You then check for the header:
If this header is not present then Peakhour is not currently serving your content. If it is present and has either cdn or cache present in its list of values then Peakhour is serving it from its cache.