Check whether user has a Chrome extension installed

Question

I am in the process of building a Chrome extension, and for the whole thing to work the way I would like it to, I need an external JavaScript script to be able to detect if

Answer 1

Your extension could interact with the website (e.g. changing variables) and your website could detect this.

But there should be a better way to do this. I wonder how Google is doing it on their extension gallery (already installed applications are marked).

Edit:

The gallery use the chrome.management.get function. Example:

chrome.management.get("mblbciejcodpealifnhfjbdlkedplodp", function(a){console.log(a);});

But you can only access the method from pages with the right permissions.