To test goggles, hold them up to your eyes without putting on the strap. Lift your eyebrows (like you are surprised) and press the goggles into your eyes until they seal. Make sure the gaskets are clear of eyebrows and other hair that might break the seal. Let the strap dangle. Now let go. The goggles should stay sealed to your face on at least one side.
Note for parents and swimmers: Goggles coming off on a dive are a technique issue, not a fit issue. It has to do with the angle the head is hitting the water - if the head is up the water breaks against the goggles and pushes them down. Keep that head tucked in! Your coach has the details.