Speak to Captain Parker at the Aerotech Office Park. You can ask about the disappearance of refugees from the area, and offer to help him find them. Parker will give more details and add a location marker to your Pip Boy map.
Enter the Casa Madrid Apartments and search for evidence on the ground floor.
Dermot's room (2nd left) contains a ledger, on a table near the fridge.
Sweetie upstairs will talk about him propping a teddy bear on the bed when using her services and it being in his room downstairs when persuaded for 200 caps (100 with Barter 30) to forget about her best client's privacy.
Saint James' room is locked (Lockpick 50). You can pick the lock (bad karma though labelled yellow), or get a key from Sweetie upstairs (by paying 250 caps, or using Lady Killer or Cherchez La Femme). After Saint James's demise you get the key from her for free (for this to be still useful you would need to kill the pair without confronting them with the evidence).
Get the teddy bear near his bed (of the 3 or 4 teddy bears in the building it is the one not marked as owned). It is needed for confronting the pair, although Saint James will not spill the beans when you show it to him.
With the evidence collected, you can report back to Captain Parker.
(Optional) You can confront Dermot or Saint James with either piece of evidence. They will become hostile and attack you. Note that other Westside residents will not get involved in the fight, and will not be hostile to you afterwards.
Whether you confront the traffickers or not, speaking to Captain Parker back at the Aerotech Office Park will complete the quest.
Although not required to complete the quest, you can speak to the other Casa Madrid residents about Dermot and Saint James.
Sweetie, Maude and Jimmy will comment on the pair, and react to their deaths.
Marco will talk about their tenancy. Should they be killed, he will complain about his lost income and the need to remove their corpses.
If you kill Dermot and Saint James, you can talk to Pretty Sarah and show her the evidence in the journal. She will show her disgust and reward you with 4 Med-X. If you show the ledger to her before you kill the pair, you will not get those.
After you killed the pair, Westsiders on the road will immediately add a random comment "We still have it better than those two slavers." to their usual banter.
pcps3xbox360 After accepting this quest, the quest marker on the compass points towards a location that is impossible to reach, making it impossible to speak to Saint James. [verified]
pcps3xbox360 In both Stage 4 and 5 in some games its impossible to turn in the quest to Parker. Sometimes finishing the conversation and starting a new one will trigger the correct response. If you find it impossible to turn in the quest on the PC selecting Captain Parker with the console open and typing resurrect or resurrect 1 will enable the correct conversation choices. On a console, if the conversation option doesn't turn up to finish the quest, simply change outfits to 'gang' related outfit, and then talk to him. Usually changing people's opinions about you restarts conversation loops. On the PC, the quest-concluding conversation option appears after entering and exiting conversation with Parker twice and then initiating conversation for a third time. [verified]