First, you may already know this, but in this
thread Jon describes the functions of the USB-UIRT - including the 56k option.
Second, what software are you using? You must have application software that uses the USB-UIRT API layer and provides a receive callback function to interpret received IR.
Another thing you can do is take the USB-UIRT apart and see if the IR detector is on the board. There are 3 screws that hold the USB-UIRT together (one may be under the label). Take these screws out and simply look to see if the 56K detector is present. On the right side looking at the front you will have a red indicator led, next the frequency detector which is small and clear in color, next will be the 56K detector if present (if not there will be 3 holes in the circuit board), next is a dark colored 36-40K detector, and then 2 clear infrared emitters.
I also believe the model number on the label will read USBUIRT0056. I do not have the 56K option and mine reads USBUIRT0038.