The default QML project has a main QML about like this:

import QtQuick 2.10
import QtQuick.Window 2.10

Window {
    visible: true
    width: 640
    height: 480
    title: qsTr("Hello World")
}

If I change the "true" to "trueffff", it becomes an undefined reference and running the project readily tells me about it:

qrc:/main.qml:5: ReferenceError: truefff is not defined

The QML code model could know that "truefff" is an undefined reference and underline it in red.