I don't think you can - this would be an opportunity for a security leak if it were possible - you'd be able to get the name of the computer and then via WSDL tell someone else the name of the computer the user is using.
You can get the platform (xfa.host.platform) and the host application (xfa.host.name, usually "Acrobat") but nothing that's particularly identifying.
But in my case I have to uniquely identify a user and allow him/her to add data one and only one time (when the form opens blank). And then have the document emailed to their manager. Manager will have edit rights. This would be offline PDF document so at form load I have to uniquely identify the user.
You don't need any fancy hardware to digitally sign a document. I've attached a PDF that's a Designer form that has a signature field on it; try it out and see what you think.