Validate user in SAML

i have SSO configured and have a requirement where in i need to input a user id in one of the text box. once i add the user id there, i would like to run a validation if that id exists in our directory database , so we know that is a valid user. if not return that user does not exist. what are the ways to achieve this solution.  
1 answers

May I ask the reason for this requirement? Because when doing the SAML login and the user can not be found in the Mendix database that user get's a notification that the person succesfully authenticated but that no valid user could be found. And a system like your requirement is imho a security risk because it is a easy way to see which usernames are valid for your app.