How did Fagin discover where Oliver was living?

Nancy and Bill searched for him (scouted).

"Do you think Nancy and me has got nothing else to do with our precious time but to spend it in scouting arter and kidnapping, every young boy as gets grabbed through you?"

Fagin sent Nancy to the police station by making here the sister of Oliver. There she came to know that Oliver was living in pentonville with a respected gentleman named MR. BROWNLOW