How do I know if I need to replace the throttle body or just the sensor?

What is the code(s) and the problems you are having with the car? As to what to replace, you normally test the parts to determine what is or isn't faulty. Then it comes down to what is actually available or repairable without special equipment. For instance, say the problem on yours was that the throttle position sensor wasn't reading correctly. That part is replaceable on its own, as is the idle air control valve, as shown in the image attached. So, for those you replace the part to repair the unit. However, on some vehicles they include those as well as temperature sensors and air flow sensors built into the throttle body that require special tools or procedures to work with. Some are even epoxied into the throttle body, for those you replace the entire unit.
However, before anything is done you need to test to determine what the issue actually is. As an example, there were GM trucks that would set codes saying the throttle body had failed. GM even issued a TSB stating, "if you have these codes replace the throttle body" However there were multiple cases of the replacements setting the same codes. It took someone outside GM to determine that the real problem was in the wiring connector on the harness that was poorly made and causing a bad connection. So, we advocate a simple idea of "Test don't guess" Especially as parts increase in price, people get upset when someone puts a lot of money in parts onto a vehicle, to have it still act the same.