Usually the easiest way to tell the difference between Overo and Tobiano is this:
Tobiano - the white markings at some point come over the back of the horse; be it mane, spine or tail.
- their head is usually the last part of them to become white (although a blaze or star is pretty common, bald and apron faces are not)
Overo - the white starts out from the bottom of the horse's body first ; usually somewhere on the belly or neck. The last part of an overo to have white is near the spine.
- bald faces are usually seen on overos.
Hope some of this makes sense and helps, but for more information, visit the APHA website (american paint horse association). They should have a link with that information.
Take care. ;-)