How can I find out how well integrated is a given airport in the national/international flight route network?
You can use Kayak.com -- Type in your departure airport on the flight search page and as destination, select "Anywhere - see where you can go on your budget" as destination
In the English Wikipedia, most airports have a list of connections offered. This allows you to check if they are major airlines connecting the airport to their hub. For instance, https://en.wikipedia.org/wiki/Cologne_Bonn_Airport contains a comprehensive list for the Cologne-Bonn airport.