How do I find out if my assets are being optimized?

By defaul, any resource going through us will be optimised (unless there is a page rule or a URL parameter like opt=none or something similar which has disabled the optimization.

Inspect the response headers of any asset being routed via Dexecure. You should see a header called "dexoriginalsize" which we send indicating that the asset is being processed by Dexecure.