What usually works best for me is talking to the person who is listing the week. Sometimes over the phone is best but you can usually tell if they own the week via email communications.
Also, if the week you want is much cheaper than all the others, odds are it is an exchange. With that said, I have yet to read a recent post from someone who rented, then went to check in and found their rental was not valid for hatever reason. Some of what gets posted here on tug is regurgitated scare tactics.