In the Dashboard (Performance > General Settings) there’s a Debug section where you can turn on debugging for all the different caches, and it adds comments to the HTML source code, so you can see which pages / objects / DB queries have been cached.
If you enable debugging for Page Cache, if it can’t cache the page it’ll say Caching: disabled and give you a reason, e.g. User is logged in (by default, pages for logged in users aren’t cached.)
Remember to turn debugging off again when you’ve finished.
Note questions about third-party plugins are off-topic here; the best place to seek support would be the W3 Total Cache plugin page.