how can I tell if my monitor is 120hzi swear it said it on the box but im looking it up online and it says nothing about how many Hz it is
Hz is a unit invented by Hertz which was the first artificial wave. Hertz indicates the amount of cycles per second. Computers have a very high amount of cycles per second, which is why they use gigahertz.
You'd need a screen with more than 200hz to test this, but most likely no.