The xp_logininfo reports the Domain groups (or other valid SQL login) how the user is being granted access.

https://msdn.microsoft.com/en-us/library/ms190369.aspx

EXEC xp_logininfo 'BUILTIN\Administrators';

EXEC xp_logininfo 'Contoso\JSmith'

Result:

account nametypeprivilegemapped login namepermission path

Contoso\JSmithuseradmin Contoso\JSmith Contoso\DBAdmins