It is all about the underlying wireless chipset the WiFi adapter uses. Read the manufacturer spec sheets for the adapter in question and see if it lists the chipset it is using. Be careful: the chipset can sometimes vary between different revisions of the adapter; make sure you look up the revision that is currently being manufactured. If the spec sheet does does not list the chipset, do some research with your favorite search engine and try to figure it out.
Once you have the chipset, find the model on the chipset manufacturer's website (likely Realtek or Atheros) and it will more than likely list the supported features and protocols. Again, if not listed, do research with your favorite search engine.