As it has been mentioned before, you do not want to check whether the device is a tablet or a phone but you want to know about the features of the device,
Most of the time, the difference between a tablet and a phone is the screen size which is why you want to use different layout files. These files are stored in the res/layout-<qualifiers> directories. You can create an XML file in the directoy res/values-<same qualifiers> for each of your layouts and put an int/bool/string resource into it to distinguish between the layouts you use.
Example:
File res/values/screen.xml (assuming res/layout/ contains your layout files for handsets)