How will the ethereum contract program know that the worker's terms are met? If someone contracts me to build a program, and I get paid when it's done, how do y

How will the ethereum contract program know that the worker's terms are met? If someone contracts me to build a program, and I get paid when it's done, how do you define done, which is even hard in real life. Programmatically, I could assign them ownership of a domain, or maybe a github repository? I don't see how in a world where finished is ambiguous, this can work. If this was an advertising or marketing contract, and I paid part of it as we go, how could a computer program contract know when milestones are reached? The only feasible way I see if the buyer has to say "milestone x reached" electronically. But if that's the triggering mechanism, then what happens if the buyer never does that? It seems no better or worse than regular contracting.