How do they know when the writing is pictographic (an idea expressed as an image, like a big predator showing teeth), or syllabic (an image of a bestial grunt, basically, like 'ugh' or 'caw' or what not) or alphabetic (the breakdown of syllabic utterances into, at first, the hard consantants and the vowels)?
Basically, the number of symbols and the repeating patterns. But it seems that in this particular case, they also relied on the shapes of the "letters" to conclude the alphabetic nature of the script.