When you do the fake booking, after you've selected the category, and click "Continue", scroll down to the STATEROOMS section, you'll see a line that says "Stateroom: Standard Inside Stateroom, Category 11C Change " (or whatever category you selected).
Click on "Change". The next page you'll find a line that says "Stateroom Number Not selected Select"
Click on "Select". You'll find a page that either says, "sorry, no rooms available in that category" or will have an assortment of available rooms. There's also a "Choose your own stateroom" option.
That's where you fill in a room number and click "Save" it will then tell you whether that room is available or not.