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