The Midwest Alliance for Mindfulness has a good introduction to the foundations of mindfulness taught by the Buddha, including the third foundation, being mindful of the mind and heart. We especially notice whether states of mind like aversion, attachment, or confusion are present or absent. Their article covers all four of the foundations of mindfulness, we’ll be discussing the third foundation, which you can find in the middle of the article. You can access their article here: Foundations of Mindfulness