The way to check if it is fertile is to crack it into a dish and see if it has a bulls eye in the yolk. There are other posts with photos of this on the forum which I will try to find for you. Obviously the egg cannot then be further incubated, but can be scrambled and fed back to the duck.
It is too early to tell by candling in my opinion especially if she has not been sitting tight for several full days and nights. All eggs have an air sac in the blunt end whether they are fertile or not. If they are fertile and they have started to incubate, they will start to develop a network of blood vessels first.
I assume you do have a drake. Have you seen them mating?
When you say, "she was sitting at first but I have not seen her sitting anymore" what do you mean? She was sitting for a few days or perhaps you just saw her sitting for an hour or so each day, whilst she was laying each day. Once she stops laying and sits day and night and does not want to get off the nest even when you try to lift her off, that is when she has gone broody. Until then, you are just wasting eggs leaving them in the nest. It might be best to get some fake eggs and leave them in a nest as having a nest full of eggs will encourage a hen to go broody, but she has to have the inclination to it in the first place.
Regards
Barbara