You may want to know whether the user is on a desktop or a mobile device. The Designer server is equipped with detection software it uses to determine whether it will server the desktop or mobile version of the Designer application.
You can poll the Designer server for with a simple API call like:
http://he11.printscience.net/pdf_admin/rpc_api_v_4_0_0/get_version/product/[productID]/
The productID you use may be the productID of any valid product in your account on the Designer server
The reponse will be a simple json string like:
{"app_type": "desktop"}
where the valid values for app_type are: "desktop" or "mobile"