By default, the ID of a post shows in the link to the post. For example, look at the URL for this very page — the ID is 52.
If you have custom “permalinks” that hide the number, you can always get the ID number of the post from the URL when when you edit the post.
It will look something like this:
The number in that string is the post ID. It doesn’t matter whether the item is ‘post’ or a ‘page’.