Many blessings to you by brother.
Over the course of my life with Christ I must admit my thoughts on this in particular have changed. If you are producing Christlike fruit and your life is a reflection of the fruits of the spirit and as you progress through life you are becoming more and more like Christ, then I think you know without a doubt. Christ taught that the wheat and the chaff would be co-mingled within the church, by church I mean the universal or worldwide body of believers which currently exists. There are weeds in the church sown by the enemy. But Christ warned us to not pull them out as by doing so we may uproot the wheat. If you think about a church one can understand how this could happen quite easily. The weeds are like those who claim to have done things in the name of Christ but He never knew them. They also knew they were not children of God but at the judgement cry out regardless. So you know if you are His. You can fool others, you can fool me, but you cannot fool God and you cannot completely fool yourself. I think your next to last statement also sheds light on this, you who practice sin. Ask yourself this question, can a follower of Christ practice sin? Not can a follower sin but can a follower practice sin. Some will say a follower cannot sin, I am not thoroughly convinced of that due to 1 John 3:9. But I don't want to turn this into that.