Signature ques type customization

I have one query regarding the extraction of input text from Signature question type.
I want to apply a read only logic depending on the input of Signature box -
var SignatureBox= jQuery("#QID8-Signature.InputText");
if(SignatureBox.val()!==''|| SignatureBox.val()!=='Sign Here')
{
q.find(".InputText").prop("readonly",true);
}
It would be very helpful if you can guide me on this issue.