'A nod's as good as a wink' is a way of saying you have understood something that someone has said, even though it was not said directly. The full phrase (sometimes used in the UK ) is 'a nod's as good as a wink to a blind horse'.
This idiom is used when describing the way a situation appears, while allowing for the possibility that things may be different:
On the face of it, the company looks very profitable. (The company appears to be very profitable, but this may not be the case.)
When someone is wearing a plastic smile, they are appear to be happier with a situation or events than they actually are. This is actually a description of the forced smile you might see in many photographs.