Inspect for consumer plans only works with Sketch documents synced to prototypes via the Craft Sync plugin. All artboards from your Sketch document will be Inspect-ready.
Enterprise users and existing Inspect Public Beta users will be able to continue to use server-side extraction (via drag and drop method, InVision sync, and Craft Sync).
You may not see the contents of your Sketch file in Inspect if the screen extraction completes before Inspect processing finishes. In that case, wait a few minutes and you should then be able to inspect your Sketch file’s contents.