Fortunately there's a new extension which can do just that. What?! An extension to peek under the hood of other extensions? Yep. It's called Extension Gallery Inspector, and it's pretty slick.
Install the Inspector and head to the Chrome Extensions Gallery. When you browse an extension's detail page, the lightning bolt icon will appear in your Omnibar. Click it, and Inspector will unpack the .CRX file and scan for API calls, permissions, and OAuth support. It also tells you the uncompressed size of the extension and whether or not it has an options page and browser action icon.
You'll obviously need a bit of technical knowledge to benefit from Extension Gallery Inspector as it stands right now. Still, for power users who want to know what an extension has access to -- or developers who are just curious how a particular extension works -- Inspector is a must-have Google Chrome extension.