You can find the Page ID in the Edit Page URL. Access the Pages menu in your blog’s Dashboard.

Then, click Edit and, in your browser’s address bar, you’ll be able to view its ID (optionally, if you place your mouse over the page in your pages listing, like the image above, you’ll also be able to see its URL in the bottom of your browser).

For example, the page ID for the following example is 2

http://example.blog.com/wp-admin/page.php?action=edit&post=2