SaveActiveDocument works for me.
It returns -1 if the document already exists and a zero if it doesn't. (And it saves the document...)
{start}
Procedure Saveme;
VAR
en:STRING;
nu:LONGINT;
BEGIN
nu:=SaveActiveDocument('testdoc');
en:=Num2Str(0,nu);
AlertInform('output:',en,TRUE);
END;
RUN (Saveme);
{end}