One of the major change of WordPress 2.5 was to remove the IDs on all admin pages. The reason of removing ID’s from admin pages was probably due to the fact that the common user don’t need them but since our templates are magazine/news style where we do have multiple loops on main pages, we will need to check category ID’s in order to set our templates.
If your browser shows the URL in the status bar, you can see the ID number when you mouse over the category link. If the status bar is disabled, you can activate it by checking the Status bar choice under View on both Firefox and Internet Explorer.
The other way of checking ID numbers is to download and install WPsite Show IDs. When you install that plugin, the ID numbers will be printed next to categories as in following screenshot.