I have a Scanner that can read live data. I am getting a P0432 error and I was told to check the O2 sensors first. Is there a way to read what they should be without just changing them in hopes that it fixes the issue?

 

The voltage from a properly functioning O2 sensor will swing back and forth very quickly in a range of roughly .3-.7V or so. You probably won't be able to tell if it is good or not by looking at the voltage on the scanner. https://www.aa1car.com/library/o2sensor.htm has a good summary of how they work and what they do. The O2 sensor after the cat is what the OBDII system uses to tell if the cat is functioning properly and if the sensor isn't reacting properly, it may flag it as a cat problem.

 

I'll add to the very good answer you already got, below.
Correct. The issue is "properly". The post cat sensors are "narrow band", meaning they jump between on and off. So if they jump between on and off in the right way, the system thinks everything is fine. The problem is, it's difficult to know whether the oxygen levels that trigger them are the right levels.
The downstream sensors are downstream from a lot of stuff: the cat, the upstream sensor, the exhaust valve, the combustion chamber, the spark plug, the intake valve, the fuel injector, the throttle body, and the MAF sensor. So what people usually should do, in the absence of other likely culprits, is fix things in the order of cost. Change the oxy sensors if they have 100k+ mi, clean the MAF sensor first. Plugs if > 100k mi. Upstream sensors if you can figure out how to get to them. Then maybe the cats.
Cats earlier if you aren't in Commiefornia or are willing to take a chance that the smog dude won't actually look under the car.

 

Our cars don't have mass airflow sensors, they have manifold absolute pressure sensors

 

Wrong

Right

but what does either of these have to do with the oxygen sensors?