Manually via WordPress
Each post, page or custom post type in WordPress carries a unique numeric ID, this ID might be used in all sorts of ways so let’s see how we can find it when we need it.
Go to your dashboard and open the post/page in question for editing, look at your browser’s address bar, you will see the ID in the post’s URL
Display Post/Page/Custom Post type IDs with a Plugin
The easy solution comes with the use a of plugin.
Reveal IDs, a great free plugin that shows the IDs of posts, pages, categories, custom taxonomies, custom post types etc!
After installing the plugin you can see the ID column in your post/page and all your listings in general.