I'm having a hard time QAing Magento full-page caching. How can I test from the frontend that a page is utilizing FPC?
One visual thing you can do to determine if FPC is serving you content if you are not technically minded is to turn on template hinting (assuming this is not a production environment). If you are being served cached content you will only see the template hints where there are holepunches.
If you are more technically minded you can use a tool like firebug to see how long the server takes to process the request for HTML. This time should be drastically lower than without caching in place (if FPC is running and processing time is not significantly lower, you may not have the best caching solution).