If the hitch is on a pick up truck, then you can almost be assured that it is a class 3/4 hitch. I say 3/4 because almost all class 3 hitches can be converted to class 4 by the use of weight distribution.
If the truck is one of the newer ones, it may even be a class 5 hitch with the 2.5 inch receiver. My son's Ford F150 came with a 2.5 inch receiver a few years ago. However, my new 2019 Ram 1500 came with a 2" one. The class is determined by the weight capabilities of the receiver.
Here is one interpretation of the hitch classes.
Here is another. . Be aware that there are others.
Barney