They literally can't manipulate the results. First you have to send your client seed with the transaction. You can edit the client seed at any time.

The server seed is updated and published each 20 blocks (60 seconds) and these 2 seeds from the blockchain are used to generate the numbers. All the technical details are presented here: https://steemit.com/steemit/@magicdice/magic-dice-tech-talk-provably-fair-and-transparent

All the scripts you need are published on GitHub which can be fount at https://github.com/magic-dice/magic-dice-report

There is no way to rig the game unless magic dice team edit information on the STEEM blockchain. If magic dice isn't fair then it would mean that STEEM blockchain is compromised which would either need someone to have tampered with the software that run the witness nodes or magic dice needs to be working together in secret with 15 of the top witnesses to edit content on the blockchain. If that happens we are all screwed. I don't see why any top witness would be taking such a risk for small financial risk from magic dice.