Display display = getWindowManager().getDefaultDisplay();
int width = display.getWidth();
int height = display.getHeight();

boolean is_tablet = false;
if (width >= min_tab_width && height >= min_tab_height) {
    is_tablet = true;
}

<manifest ... >
    <supports-screens android:requiresSmallestWidthDp="600" />
    ...
</manifest>

//Determine screen size
if ((getResources().getConfiguration().screenLayout &      Configuration.SCREENLAYOUT_SIZE_MASK) == Configuration.SCREENLAYOUT_SIZE_LARGE) {
    Toast.makeText(this, "Large screen",Toast.LENGTH_LONG).show();

}
else if ((getResources().getConfiguration().screenLayout &      Configuration.SCREENLAYOUT_SIZE_MASK) == Configuration.SCREENLAYOUT_SIZE_NORMAL) {
    Toast.makeText(this, "Normal sized screen" , Toast.LENGTH_LONG).show();

}
else if ((getResources().getConfiguration().screenLayout &      Configuration.SCREENLAYOUT_SIZE_MASK) == Configuration.SCREENLAYOUT_SIZE_SMALL) {
    Toast.makeText(this, "Small sized screen" , Toast.LENGTH_LONG).show();
}
else {
    Toast.makeText(this, "Screen size is neither large, normal or small" , Toast.LENGTH_LONG).show();
}