Open a xml layout file:
- at the bottom, you have two tabs: Design/Text
- in design mode, you see the preview directly
- in text mode, the preview is (by default) on the right, toggled by a 'Preview' button in the rightmost bar
You can also show/hide it with View > Tool Windows > Preview
EDIT
Attention: the IDE only displays the preview if editing a layout file in the res/layout* directory of an Android project. In particular, it won't be displayed if editing a file in build/res/layout* since those are not source directory but output directory.
The Resource folder is set automatically, and can be viewed (and changed) in Project Structure > Modules > [Module name] > Android > Resources directory.
