QtQuick, how to know if a application was compiled on debug or release mode?

At Qt/C++ there is QT_DEBUG define macro to know when it is compiled at debug or release.

Is there any method to know if is the application running in debug o release mode inside a QML file?

You can use context properties (or QQmlApplicationEngine::setInitialProperties() since Qt 5.14) to expose C++ objects to QML:

#include <QtGui/QGuiApplication>
#include <QQmlContext>
#include <QQuickView>
#include "qtquick2applicationviewer.h"

int main(int argc, char *argv[])
{
    QGuiApplication app(argc, argv);

    QtQuick2ApplicationViewer viewer;
#ifdef QT_DEBUG
    viewer.rootContext()->setContextProperty("debug", true);
#else
    viewer.rootContext()->setContextProperty("debug", false);
#endif
    viewer.setMainQmlFile(QStringLiteral("qml/quick/main.qml"));
    viewer.showExpanded();

    return app.exec();
}

main.qml:

import QtQuick 2.2

Item {
    id: scene
    width: 360
    height: 360

    Text {
        anchors.centerIn: parent
        text: debug
    }
}

It's not possible to determine this purely from within QML.