We are sure that the Quran we have today is the same one that was revealed upon the prophet because the Quran was memorized during the time of prophet. Many Muslims had memorized the Quran including Imam Ali. It was passed on where thousands of Muslims memorized it, including the Imams of the Ahlulbayt.
The chances of the Quran being changed are very weak because the Quran was and is recited everyday by Muslims. Furthermore the Quran was written during and after the prophet. Yes the dialectical markings were added later but it was orally transmitted and memorized, the markings were added to help non- Arabs recite the Quran.
If there would have been a change to the Quran people would have noticed.