In most cases, evidence is very difficult to collect. The management 101 by HR is to teach managers how to protect managers themselve and the company from lawsuit. When I was in management first time, my management 101 was:
1. do not talk anything arguable in writting.
2. if you have to talk, invite employment to your office on 1:1 talk in closed door.
3. phone call is fine since there is no record of phone commmunication.
it is very tricky to prove since they know not to leave any trace of evidence from the very beginning. Due to this matter, many lawyers go class action since it is easier to prove a pattern than indidual hard-core evidence. To file a class action, one needs at least 6 people by law. However, chinese people do not unite in this matter usually.