I don't know if there's a fixed rule. If you search on the hotel site, you might see "Sold Out", which often just means they have reserved spaces for the possible conference business. If you keep looking and the "Sold Out" is gone (or you start seeing the hotel offered by other brokers), that's a possible sign that it might have gone to Priceline. Alternatively, if you see a sudden rate decline, that might mean that too. But no way to tell that I know of.