And how do you know that?

An easy way to test if it is really 120fps is frame tearing. If it is operating at 120fps, it would be able to display fluid motion all the way to 120fps without experiencing any frame tearing. So get a game where you can put a frame cap on, and set it at like 110 or 115 fps, and move the mouse around really fast and see if there is frame tearing. If there is, it is not 120Hz.