There is no way to know if a bluetooth adapter is available

bool isBluetoothAvailable1()
{
    return !QBluetoothLocalDevice::allDevices().empty();
}

bool isBluetoothAvailable2()
{
    QBluetoothLocalDevice localDevice;
    return localDevice.isValid();
}

bool isBluetoothAvailable3()
{
    std::shared_ptr<QLowEnergyController> created( QLowEnergyController::createPeripheral() );
    return created && !created->localAddress().isNull();
}

bool isBluetoothAvailable4()
{
    std::shared_ptr<QLowEnergyController> created( QLowEnergyController::createCentral( QBluetoothDeviceInfo() ) );
    return created  && !created->localAddress().isNull();
}