Hi! As Greenozaur mentioned, the Inspect feature is a new feature in the new Axure Cloud, and is accessible in prototypes that were published from Axure RP 9. To use it, open app.axure.cloud in your browser, navigate to your Axure RP 9 prototype, and hover over the page tile in the “Overview” tab; this should present the option to either “Preview” or “Inspect”. Clicking “Inspect” will take you to a view where you can click and hover elements to see useful information about dimensions, positions, etc…
Hopefully that helps, and feel free to send any feedback over to support@axure.com!