What devices measure EPR & RPM on a turbofan engine?
RPM is measured by a device called a tachometer. In a jet engine often this will be an inductive proximity probe that is measuring a toothed wheel on the shaft.
EPR is measured by comparing two static pressure sensors, one at the inlet and one at the propelling nozzle discharge, and taking the ratio. There are many different types of these. Many common ones will have a flexible membrane or diaphragm of some kind. Under pressure the membrane deflections and then you use some kind of deflection measurement.
These types of sensors are very common in many applications besides just jet engines. A little research on the internet will show you lots of details about how they are constructed and how they work.