How do I know if I have Sanctifying Grace?
Sanctifying Grace is Received in Baptism, lost through mortal sin, and regained with a good confession. A thorough examination of conscience like this one is helpful. A sin committed involving grave matter is only mortal if full knowledge and consent are present.
As stated in the Council of Trent, a person can have a "high degree of moral certainty" of being in a state of grace.