Active Sessions at Browser/REST Deeplink/Pusblish REST Service

Hi all, I am using 3-step services with REST Deeplink module for external APPs Integration. At first step, I want to check the user logged in Mendix App or not, then want  continue other steps. At first service operation, I need to check the user logged-in/has session at same browser or not. If not I want to open sign in page. I do not want to show browser default pop-up, want to show custom log-in page After log-in, continue operation and other 2 services. I do not want to terminate the user to log-in again from webservice call, if the user already logged in the same browser. Do I need session here? How can I reach current sessions at the browser? It is stored browser cookies? When I open my Mendix app url in a new tab, I do not need to log-in if I already logged in App. I want to make same situation for REST. Many thanks.
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