From a remote server I wish to see if a user is logged in to the application.

I have set up the REST API but I don't know what method to call to simply check if a user is logged in or not.

I should clarify,

I want to check if a user is logged in using the same web browser. So lets say the master application is located:

app.mysite.com

and the slave application is

mysite.com.

I want to be able to check from mysite.com if a user is signed into app.mysite.com

About the only reliable option there would be to check the sessions table for that member ID, but it's not exposed via the REST API. 

The REST API isn't designed, or intended, to authenticate or check for authentication. That's what IPS Connect or SSO's are for. 

30 minutes ago, Aiwa said:

About the only reliable option there would be to check the sessions table for that member ID, but it's not exposed via the REST API. 

The REST API isn't designed, or intended, to authenticate or check for authentication. That's what IPS Connect or SSO's are for. 

I've tried using:

/* Require the init.php file from the Community Suite root directory */
require '/path/to/suite/init.php';

/* Initiate the session to verify who this user is */
\IPS\Session\Front::i();

/* Print the user's name */
print \IPS\Member::loggedIn()->name;

However, it doesn't work across subdomains.

You can't do this via the REST API at this time. The REST API is a server-to-server API and has no knowledge of the user that may trigger it to run.

Your sample code would work, however you would first need to adjust your Suite installation to set cookies across the parent domain if it's on a separate subdomain.