jase/karen.v1.sti

recently had probs with my gearbox,its a long story so all i want to know is, is there anyway i can find out if he(garage in question!) has put another gearbox in my car? i know all gearboxs have id numbers on them, but is there a way i can match it to my car! the gearbox has been in & out of my car about 5 times, the garage says this is a second hand gearbox but i think its my original 1. can anyone help me? cheers