Yeah, like in my first answer. You read the data from the database. When the response is empty, in extDB it lokes like this i think: [1, []] you can pass an error to the client and kick him from the Server.
When you get any data(expect you got an error then you got no 1 at the first place) in the array( [1, ["My Player ID"]] the user is in the database so you can pass an success to the client over remoteExec.