We don't read the script (if it is a script - there's some debate). Basically that leaves interpretation up for grabs. I only know this civilization through TAing ancient history courses (mods, be kind!) but I will tell you what we teach (out of Worlds Together, Worlds Apart - a terrible textbook if you ask me - and don't, please I can rant about at least two major errors even though I don't have the text anymore; the pain was so scarring).

The evidence is nifty, apparently planned cities with more or less uniform houses with advanced plumbing and a citadel/religious center usually involving a bath, and a lot of things that look like seals with an animal and something that looks like writing.

If you're the sort that's looking for an ancient egalitarian society, the more or less uniform houses look like there's a lack social status, and the big bath might just be a big public bath. The citadel could be just a civic building like town hall.

If you're someone inclined towards finding social hierarchies, the existence of seals might imply the notion of private property - seals are for stamping your name on things. Why stamp your name on things if it's not yours? The bath could be a ceremonial bath which would might indicate some sort of religious hierarchy - a priest king or something?

But then we only assume the things are seals. Who knows what they were?

So in my opinion, no there is no good evidence without [EDIT: readable] writing. But that doesn't stop people from making claims.