How can I detect which layout is selected by Android in my application?

You can set a different android:tag attribute on the views in each different resource file, and read the tag back at runtime with View.getTag().

Example:

layout-xlarge-land/my_act.xml

<View
    android:id="@+id/mainview"
    android:tag="xlarge-landscape"
/>

layout-xlarge/my_act.xml

<View
    android:id="@+id/mainview"
    android:tag="xlarge-portrait"
/>

MyActivity.java

String tag = view.getTag();
if (tag.equals("xlarge-landscape") {
    ...
}