There are many plugins available that will give you the last login time for users, but what if you just want to run a quick script to see user activity on your site?
If you have access to the cPanel of your website, you may also have access to the phpMyAdmin, open up a SQL Editor tab.
Paste this into the SQL Editor and click Go
SELECT user_id, wp_users.user_login,
FROM_UNIXTIME(substring_index(substring_index(meta_value, ‘”login”;i:’, -1), ‘;}}’, 1)) as last_login
inner join wp_users on user_id = wp_users.ID
You may need to change the table names from wp_ to wpXXX_ where XXX is the secure part of your database schema.
I hope this helps!