If you get a good look at adult Violet-green and Tree Swallows, they are easily differentiated. In the Violet-green, the white on the cheek extends above the eye and the white flank patches extend to the sides of the rump in contrast to the Tree Swallow which shares neither of these characteristics.

On juveniles these marks are not easily seen, making identification more difficult. One can often identify juveniles by noting the adults from which they seek food.

Stanford Note: Although photographed at the Jasper Ridge Biological Preserve, these birds are readily seen at the Dish and at Lake Lagunita. In flight, Tree Swallow climb with several rapid wing beats and periodic glides while the smaller Violet-green glides less and flaps more quickly. While tending nestlings, these aerial foragers make about 350 feeding visits for a brood of five, bringing up to 8,000 insects.

For additional photographs of Jasper Ridge birds see:
http://www.stanford.edu/~petelat1/birdlist.html#jasper