[w] How can I find out if a Contracting State to a particular Convention has extended the application of the Convention to one or more of its territories?

First click on "Conventions" and select the Convention you are interested in. Then press the button "Status table" in the right-hand navigation menu.
Where you see a figure in the column "Ext", the corresponding State has extended the Convention to its territories. Clicking that figure will lead you to the table of extensions. This table shows to which territories the application of the Convention has been extended, the dates of these extensions and the dates of entry into force of the Convention for these territories, as well as any declarations or reservations that may have been made, and authorities that may have been designated for these territories. There is also, for some Conventions only, a table of acceptances of extensions available.