The fact they refer to Gardulla as "she". If it was as the EU claims, Hutts would be correctly called "it", or at least "he" all the time.
Also the way Anakin speaks of her, it sounds as though Gardulla was both female at the time of the Phantom Menace and female when she lost Anakin and Shmi to Watto, which is unlikely if their sex-changing is as described.
Also, in the book it describes Hutts in their female form as much smaller and differently coloured than male Hutts. This was an attempt to justify the poorly made CGI of Jabba in ANHSE: they tried to explain it be claiming Jabba was in a female form at the time he confronted Han. The different colouring theory at least was disproven by Gardulla's appearance in TPM, and indeed Jabba's renewal in ANHDVD also does away with the messy theory.