Many of the specific techniques machines use to detect counterfeit money are closely guarded secrets, since once counterfeiters know how the machines work, they try to foil them.
But two common means of identifying paper money are magnetic sensing and optical sensing.
Inks used to print notes have certain magnetic properties, which can be sensed by a magnetic head in a bill changer.
A computer can recognize the electronic signature of specific tracks along the money and accept or reject it.
In optical sensing, light is passed through or reflected from a bill. Some inks absorb light and others reflect it.
The computer can analyze these patterns and determine if the bill is acceptable.
In either case, the scanning of the bill takes about one second.