How to know what serializer_class is used

I define a serializer class in the get_serializer_class function. In my update function, how can I know which serializer class is selected? What is the best practice for this?

Simple code:

class SomeViewSet(ModelViewSet):
    model = SomeModel

    def get_serializer_class(self):
        # here I define my serializer class, let's say between FirstSerializerClass and SecondSerializerClass

    def update(self, request):
        if self.serializer_class == FirstSerializerClass:
            # I checked this way, but it didn't work for me