At the moment the only way to detect it is by checking the Application's width and height. For instance assuming that the phone's widthxheight is 40x71 GU (default portrait), the tablet is 160x100 GU (default landscape), anything that goes above 160x100 GU can be considered as desktop.

There are plans to provide flags to identify for instance whether there is a HW keyboard attached, pointing device, ASO, however those flags should not be used alone to identify a desktop form factor as you can attach mouse/HW keyboard to phone/tablet through bluetooth.