Getting "WhoAmI" from a domain?

I VPN to multiple domains, but a specific application I'm writing is only is
conserned with ONE domain.

Is there a way to query that domain to find out the user name logged in as?
I'd like to write a function like:

function WhoAmI ( domainName: string) : string;
begin
//real stuff goes here to get the user name

  result := 'logged into ' + domainName + ' as ' + UserName;
end;