On this page of the game guide to Borderlands 2 you will find a detailed description of the side mission titled BFF's. You can start this quest in the Sanctuary location after completing the main mission named Where Angels Fear to Tread.
1) Sam Matthews
In front of Scooter's garage you will see four NPCs aiming at each other . Accept the mission of determining who's the thief from Sam. Talk with all the NPCs for additional experience points. It however won't give you any substantial evidence. Aim at any NPC and shoot at him/her (I'd recommend Jim). Afterwards finish the mission at Sam.