Assuming you're referring to Server license and assuming you're an Admin, on your Server page, you might've already looked at this but there should be a "Users" tab. Click on that and it should have list of all the Server/Site Users, user names, roles and last logged in information. Sort "Last Logged In" info to identify users.
I'd recommend taking advantage of Matt Coles efforts: Tableau Server Insights (formerly Shareable Data Sources for Tableau Server)
His work takes care of all of the connections and dimension/measure labeling.
Thanks, yes I'm and admin and i know that server last login info we get to see under user tab but still if I wanted to cross check this info with my backend data which gets save in postgres. It happens many a times that user name is there but all login information is not there let me show you in screenshot
here in backend Tableau is not catching every login date as you can see in login_at column first 3 rows data is missing.
That's the reason I'm not sure if tableau is giving correct data.