Someone who firmly supports the neo-SSPX, and hates the Resistance, asked me on Facebook, "What if you have fallen from grace?"
There would be evidence of it. Even if we are never 100% sure of where we stand before God, nevertheless we can have some idea, based on our spiritual life, prayer life, how often we fall into sin, etc. Plus it's a question of Free Will -- what do we want? God or ourselves?
Do we want to deny ourselves and follow God, or deny God and follow ourselves. If a person looks deep within himself, he will know which camp he's in. It's not a complete unknowable mystery.
If you are willing to make any sacrifice to follow God, then I seriously doubt God will cast you into hell, even if you turned out to be objectively on the wrong side. We can only be expected to follow our conscience.
It's a question of following God's will, to the best of one's ability. When it comes to knowing God's will, the Ten Commandments are always a sure thing. Also the 6 Commandments of the Church.
Beyond that, sometimes God's will for us isn't so clear-cut. We have to use the virtue of Prudence, which means we might get some things wrong. What's important is where we place God in our hearts: First, last, or somewhere in between.