Is there anyway to read the fuel level via a multimeter, by resistance or voltage?
How about any PID value?

 

The stock sensor in the tank is just a variable resistor to ground. High resistance (40-50 ohms) means the tank is empty, becomes almost zero ohms when full. It's not going to be much mathematically proportional to the number of gallons in the tank, it just goes from "F" to "E" as the tank empties.

 

Do I just measure the yellow/black wire in the fuel gauge in the cluster?

 

Yes, of course you have to unplug the cluster or take the gauge out so that it is not in the circuit. As I said, there is one wire going back to the tank that provides a resistance to ground which directly drives the gauge on the dash. It isn't connected to the ECU or anything else.