The Indus script remains to be deciphered and we will probably never be sure until a bilingual text of some sort is found (not improbable). Until then, Asko Parpola's work on the fish sign is probably the closest to guarded approval by other major scholars. The initial insight into the fish/star connection in Dravidian languages was made by a Jesuit priest, Father Heras, in 1932. Dr. Parpola has been studying this undeciphered writing for over 40 years at the University of Helsinki in Finland and is co-editor of collections of all seals and inscriptions in India and Pakistan.
More detail on his work and proposed decipherments can be found at Deciphering the Indus Script.
See also: Mahadevan's Indus Script Dictionary.