Russ,
You display it the same way that the amazing JRRT wrote it: You make him
pause and consider whether he will persue the attack. I can see no reason to
believe it wouldn't work as well visually as it did in print. Subtlety works
in film, even if most filmakers do not understand the concept.
Tim
I guess it's hard to visually display doubt with something invisible save
for a cloak and crown. Maybe in the extended version we'll hear him mutter
"oh, bugger, it's a girl!" or something. 
Russ