I have zero modding experience, so I cannot say for certain the following is correct:
The same trigger that starts the dialog with the thugs (exchthugs01.utt) is the same trigger that checks if they're dead. When you enter the trigger it will initiate the conversation (tr_exch_thugs_01.ncs). When you exit it checks to see if they're dead (tr_exch_thugs_ex.ncs). If they're dead the quest updates (journal entry "InterGRU"). There's also a global number that should be updated (302NAR_ExchThugs_01), but it's not listed in @Hassat Hunter's globals thread so I don't know what it does.
So, you just have to enter then exit the trigger. I have no idea how big it is, I don't know how to read the coordinates.