The easiest way to determine the frequency your reader supports is by looking at the model number.  

Model numbers beginning with a 6, for example RDR-6081AKU, are Low Frequency, 125kHz readers.

Model numbers beginning with a 7, for example RDR-7081BKU, are High Frequency, 13.56MHz readers

Model numbers beginning with an 8, for example RDR-80581AKU, are dual frequency readers and will support both Low 125kHz and High 13.56MHz frequencies.