There are 3 ways to get into the slums before Echo:
- get caught by the entrance guard (dialog option or be nude)
- using the meta knowledge
- have the badge (mansion barkeeper quest)
Talk to the Barkeeper in the Mansion (nude) then you can find the Badge in the Locker Room.
- ng+: get caught in the beach house
Not being nude allows it to talk to a few NPCs.