Comment 2 for bug 1610929

You can already do this by querying Unity 8 GSettings (qml snippet below):

import GSettings 1.0

GSettings {
   id: settings
   schema.id: "com.canonical.Unity8"
}

...

if (unity8Settings.usageMode === "Windowed") {
   // running in windowed mode
} else { // == "Staged"
   // running in staged mode
}