A number of versions ago, WordPress has stopped showing category IDs in the Admin area. However, there’s still a way to find these ID’s.
Login to the admin section, then go to Posts > Categories. After that click on the title of the desired category.
Then look at the site url. It should be something like: wp-admin/edit-tags.php?action=edit&taxonomy=category&tag_ID=5&post_type=post.
tag_ID=5 – that’s your category ID.