Earlier this year I started getting long bookings (for me 4 days is a long booking) for August. Since I keep reading here of fake bookings I tried to verify the bookings. These are some of the things I did over the past months: (1) Send messages with the usual welcoming letter but I also asked for things like flight numbers. In one case one man who booked a one week stay told me that he hadn't booked his flight yet so I asked him but will you be staying at my place? His answer "maybe". I told him there and then to cancel the booking which he did. (2) More recently I sent messages to those who booked 3 days or more saying that I had a special free offer for them: free airport shuttle or a free excursion but that I needed flight details.
From the way they respond you can tell whether they are genuine bookings or not.