Dianthus seeds come in a little torpedo shape seedpod inside the calyx. When the seeds are ripe, the seedpod splits at the top and opens up in a little star shape. It's long and thin, so sometimes you have to help the seeds at the bottom out. The good seeds are black, flat and round, but sometimes there's some unripe pale brown ones or smaller bits in the pod.
After you've collected them, put them somewhere dry for a few days before you store them. I keep mine in paper envelopes in a lidded plastic container in the fridge. Remember to label them straight away!