Once a project is complete, you can rate the specialist on a few dimensions (communication, quality, and value) and leave a public review of their work. The specialist doesn't see your individual ratings; they only see their overall averages. Public reviews show up on their profile for other buyers to read.
Checking out a specialist before you hire
Every specialist's profile shows their stats: how many requests they've received, their response rate, how many projects they've completed, and their average ratings once there's enough data. You can also read recent public reviews right there.