Since WP version 2.5 it is not easy to find Category ID used in your WordPress sites.
Category ID is different from category name.
Sometimes you need it to exclude or include certain categories from some applications.
Here are two methods to help you find hidden WP Category_ID instantly.
First trick is to go in WP admin to Posts-Categories and hoover your mouse over category name - category ID will be revealed in status bar at the bottom of the page.
The second method is to use WP plugin which will show hidden IDs in admin menus, like Posts - Categories.
This plugin will reveal also other hidden WP admin IDs.