You can't. And you shouldn't trust some John Doe and deliver the full solution upfront. I personally look for long-term tasks from "legitimate" companies. After some initial collaboration, you and the company naturally begin to trust each other. E.g. I'm paid hourly, but I don't use any control mechanism for that (like that odesk screensnapper and keylogger, ridiculous...). I just state how many hours I worked for some task and that's it.

If I were a web developer, I definitively apply for many remote web dev jobs you can find on HN's Who is Hiring thread. Try next month - and double your odesk rate.