Can I check if a user has logged into the app?

Yes – To do this, you would need to go to the General Reports section, under the Reports tab. Click on “more details” at the bottom of the columns, you’ll be taken to a new page with more details and a Status column on the right side. 

“Itinerary Published” means that the itinerary has been published, but not opened.
“Itinerary Opened” means that the user logged in. This applies both to the web view/online itinerary and the app.