I'm not sure if I'm blind here, but short of digging directly into the database tables, where would I find the database id for a post or a page?
IDs always show in links in admin area, like /wp-admin/post.php?post=2164&action=edit
There are also some plugins that simplify this, quick search came up with Simply Show IDs.
Also, the status bar at the bottom of your browser is usually a convenient place to note the id when hovering over a link from within the admin
Another plugin that can do this (and one I use often) is "Reveal ID for admins" http://wordpress.org/extend/plugins/reveal-ids-for-wp-admin-25/
If you're a code geek, you might want to add
<body <?php body_class('class-name'); ?>> and your body tag will show the page-id