In non-breeding plumage distinguish it from the rare Glossy Ibis by the color of the facial skin and eye. The eye on a mature White-faced Ibis will be a bright red. Its not clear from the liturature when the eye changes color, but most agree by January all White-faced Ibis will have a red eye.
The facial skin on the White-faced Ibis will be pinkish, the Glossy will have dark skin, with a light bluish edge of skin, not feathers. The color of the legs has been given as a fieldmark, but in my experience its not very useful. The color diffrence is in the joints, and they are usually under water.
Immatures can not distinguished.
White-faced Ibis frequent shallow muddy marshes where they can probe for food with that long decurved bill. I don't see them as often in flooded rice fields as White Ibis, they tend to stay more in marshes with lots of vegitation. I can't say for sure that I've never seen them in salt water, but it must be rare.
Look for the White-faced Ibis at Anahuac National Wildlife Refuge
A breeding range map is available form the Breeding Bird Survey
A winter range map is available from the Christmas Bird Count