How do Medical Representatives operate?

Just curious…
When MRs reach out to doctors and offer them incentives to prescribe their medicines, how do they track if the doctor is actually prescribing their medicines? Is it purely on faith?
In the case of online pharmacies where people upload their prescriptions, it is obvious to track who is prescribing what.
For offline stores, one possibility I can think of is that people generally buy medicines in the vicinity of the clinic/hospital, i.e. they go out of the clinic, find the nearest pharmacy and buy the medicines. Do the pharmaceutical companies track the sales trend in the vicinity of doctors?