At what point is a contract implied?

It depends what state you are referring to, as contract
law is state law for the most part, and it further depends on what judge you are sitting
in front of, as they tend to interpret implied agreements somewhat
differently.

But in general, definitive action on the part
of both parties involved counts as evidence of an implied contract.  For example, if
party A writes a check to party B, and party B then delivers services or goods to party
A, then the law would assume an implied contract existed, whether it was written or
verbal and whether or not written or recorded evidence exists.  This is because the
court can reasonably assume intent based on the actions of the
individuals.

If I go into a restaurant, sit down, order
food and it is served to me, an implied contract exists in that I have agreed to pay for
the food, whether I said so or not.  It would not become an implied contract until the
service was given to me, since businesses have the right to refuse service if they
wish.  I wouldn't have any contractual rights, implied or otherwise, until they agree to
serve me.