The project that I am trying to create is a system to measure the RPM of a DC motor using one or a combination of the following; voltage, Current, or Back EMF.
One signal that I have looked at is the Voltage and there are spikes in the signal, is there a code that would be able to count the number of spikes in the signal then display that number on a 4 digit 7 segment display?
Another option that I have looked at is the current signal, this signal I am able to find a peak to peak frequency, but I am not sure how I would write a code to display this on the same display as mentioned above
The third way that I have looked into so far is by using back EMF to read this RPM there is this link explaining the process, however again I am very new to Arduino and unsure how to make this work and display the RPM onto the display. AB-021: Measuring RPM from Back EMF - Precision Microdrives
Measuring the motor rpm and displaying it on a 7-seg display is two different problems. Break it down. Get the rpm measurement working by printing the result to serial monitor. Then think about the display. The code to display the value will be very different depending on the type of display you choose. Some displays may have one of several common driver chips. Others are bare modules which you will need to build a driver circuit to use (the Arduino cannot drive these displays directly because of the current flowing in the digit common connections).
The most commonly used method, to measure RPM, would be attaching some magnets to the rotating things and a Hall sensor to register the turnings. All of Your suggested approaches calls for much more theoretical and filtering circuitry and will use at least one input as well as the Hall sensor.
This link describes using back-emf and includes some Atmega 168 code.
For a small DC motor in an 00 Gauge model train I put a small black plastic disk on the motor shaft with a white blob of paint on it. I then used a QRE1113 reflective optical sensor to detect the passage of the whilte spot and calculate the time for each revolution. The code in this link is derived from that project.