Currently, non-opaque windows can be detected with the following check:

    Color bgColor = window.getBackground();
    if (bgColor == null || bgColor().getAlpha() == 255) {
        // opaque
    } else {
        // non-opaque
    }

Another option is to use AWTUtilities class. However, it resides in com.sun.awt package and is not pretty "official". It would be fine if we can introduce a new method in Window class to handle this task.