How can you tell if crowns fit properly? I can feel air between some of the teeth & they don't fit flush under the gums.

How do you feel air between some of your teeth? That is unusual. Crowns can't fit flush under the gums, you have to be able to floss around them. You still have the stump of your own tooth with the crown attached to it.

If you feel no discomfort while eating, then they fit properly.