How to get to know the state of mind
"clear darkness with peace of mind" is an experience of Samadhi. It's neither Sothapanna nor Sakadagami. When you attain Sothapanna, it makes a permanent change in you. The self view, attachment to rites and rituals and doubt are removed permanently. It's not any kind of peaceful feeling that you get while meditating. Same goes for Sakadagami. When you attain it your craving towards sensual pleasure and aversion are permanently weakened regardless of if you are meditating or not.