I assume that, since th signature field is a container field the "sign here" text is merely a text object sitting over it. If this is the case, you could use conditional visibility to hide the text object when the container field is not empty.
Thanks for Reply !!
Actually I am asking about default text, that appears in the signature dialog(on iPAD). Not under field.
Nevertheless I would investigate as I suggested. The text, even if it is apparently "in" the signature location, must be a separate object to the signature container itself.