The only really accurate way to determine whether the hotel is sold out is to visit the hotel's own website. It might show no available for the day you want to check in, however it might happen due to a hotel requiring you to either book a certain number of nights or check in on certain days (i.e. no Saturday check in).
Also, it is not too uncommon to get a hotel on PL that is otherwise soldout. It had happened to me twice already. Hotels might have preblocked a couple rooms to sell on PL or HW.