The walk and turn test in particular has eight validated clues. They include raising the arms for balance, stepping off the line, and having their heel touch their toe. So in order to capture the heel touching the toe, you'd have to have a camera dedicated to watching that, because you're looking at something where a miss would be three-quarters of an inch. If you're watching the feet, you won't be able to see the hands. You might catch the person stepping off the line, but again you have an angle that will catch one thing but not everything.
On June 19th, 2007. See this statement in context.