Live Link Face: How to detect when the face is out of the camera?

I’m following the Recording Facial Animation from an iPhone X tutorial capturing and presenting my face movement successfully.
But when my face is out of the camera, the pose in the editor will stuck in the last frame. How could I detect the lost of live animation data so to play a prerecorded animation?

Thanks in advance!

I have figured it out. Just use ILiveLinkClient::IsSubjectValid(SubjectName); method from the LiveLinkPlugin source in c++ or export it to Blueprint:

    class DAZTOUE4_API UMyBlueprintFunctionLibrary : 
    public UBlueprintFunctionLibrary
        UFUNCTION(BlueprintCallable, Category = "MyVirtualCharacter")
        static bool IsLiveLinkSubjectValid(const FLiveLinkSubjectName SubjectName);

    bool UMyBlueprintFunctionLibrary::IsLiveLinkSubjectValid(const FLiveLinkSubjectName SubjectName)
        IModularFeatures& ModularFeatures = IModularFeatures::Get();
	    if (ModularFeatures.IsModularFeatureAvailable(ILiveLinkClient::ModularFeatureName))
    		ILiveLinkClient& LiveLinkClient = ModularFeatures.GetModularFeature<ILiveLinkClient>(ILiveLinkClient::ModularFeatureName);
		    return LiveLinkClient.IsSubjectValid(SubjectName);
	    return false;