Check user authentification in Open Social gadget from backend

i am writing a gadget for an OpenSocial site (www.graasp.eu). Inside my gadget i open a connection to a server. To be exact a vert.x event bus opens a connection to my Vert.x backend with the help of sockJS.

Now i want to somehow check, if the user, who opens the connection, is succesfully logged in to the opensocial site to permit the connection.

Any idea how i can do that? Can i access the oauth token, send it to my server and ask the opensocial site, if this token is valid?

Thanks in advance.