I don't think this is a problem that can be solved via a technical solution to be honest. Personal recommendation can go a some way, but being able to vet developers for a specific role is something only the prospective employer can do.
Someone may be good at bashing out CRUD rails apps day and night and so anyone who uses them for this task is going to highly rate them, but that doesn't mean they're a good fit for your company, especially if you don't actually know what you need them for at the point of hire (especially true for non-technical founders).
Couple that with the fact that there's no motivation for 'better' developers to sign up to these services (as good devs are normally stacked up with work and have an existing network to hook into) and all you're left with is something similar to an odesk clone.
Someone may be good at bashing out CRUD rails apps day and night and so anyone who uses them for this task is going to highly rate them, but that doesn't mean they're a good fit for your company, especially if you don't actually know what you need them for at the point of hire (especially true for non-technical founders).
Couple that with the fact that there's no motivation for 'better' developers to sign up to these services (as good devs are normally stacked up with work and have an existing network to hook into) and all you're left with is something similar to an odesk clone.