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.
A contract can be considered implied when either party begins to act on an agreement made or accepts their part of an agreement.
For example, a potential tenant meets with a landlord and the landlord agrees to reduce the deposit if the tenant does some yard work on the property. As soon as the tenant begins working then the contract between the landlord & the tenant is enforcable even if a lease has not yet been signed.
Another example could be between person A & person B. Person A wants person B to do something and verbally tells person B this. Person B buys materials for the project and has acted based on the verbal agreement.