When my check engine light came on, i took the car to autozone for code reading and it read defective oxygen sensor, my mechanic just told me to change the front one, he was not sure which one is defective( there are two of them), how do i know which one is defective to avoid just wasting money?

it should have told you which 1 was bad. if you changed 1 of them and the light was still on, then it must be the other 1.