We know that through our intellect and the Qur’an/established Sunna of the Prophet (s). Everything we believe must have a basis in Qur’an/authentic Sunnah.
One of the most pressing points is the issue of the caliphate. Sunnis claim the Prophet (s) didn’t appoint anyone and left it to the Shura. But this is impossible. Please see this if you have time:
Can I ask you what led you to give up 90% of your Shia beliefs? What Shia belief do you find problematic?