Using is_user_logged_in in non WordPress Application

I refer to the technical topic;
https://developer.www.ads-software.com/reference/functions/is_user_logged_in/

This is ok for pages contained in wordpress. If i use wordpress as my security platform for logging into a php application , how can i check if the user had logged into wordpress when starting the php application which is not in wordpress?

Any help / guidance given will be highly appriciated.