Both employees and employers can review each other.
Once you have had a conversation with an employee/er on-site, you will be able to leave them a review that reflects your experience.
A conversation means you have sent three messages to an employee/er and received two replies from that employee/er, or you have received three messages from an employee/er and sent two replies to that same employee/er.
For more details, see, How do I leave a review for someone?