[Razor12911]

You need to open the file with the intent to both read and writing to it, if this operation fails then it means the file is in use.

function IsFileInUse(Filename: String): Boolean;
begin
  Result := False;  
  try
    with TFileStream.Create(Filename,fmOpenReadWrite) do // this should fail
    try
    finally
      Free;
    end;
  except
    Result := True; // this handles the exception and tells you the file is in use
  end;
end;