There are three simple ways how to check which version of WordPress you are using. If you have a WordPress site, it will be certainly useful for you but it’s not a necessary thing that you need to know.
Find the file wp-includes/version.php
This file contains version information for the current WordPress release since WordPress version 1.1.0.
Faster way, that will give you the version is a meta tag. Unless someone to delete this, you should be able to view source code (ctrl+u) on the site and look for this meta tag:
<meta name="generator" content="WordPress 5.1.1" />
TIP: ctrl+f -> find “generator”
Admin Panel Dashboard
On the Dashboard in Admin Panel, you can find a box called “At a Glance“. There you can see the version of the WordPress installation.