Headinhands,
You ask how I knew.
You're right the Holy Spirit had not sealed me because that happens the moment you first "believe in Him" (John 3:16 "for God so loved the world that He gave His only Son so that who ever believes in Him should not perish, but have eternal life".
From an atheist point of view, I read a lot about evolution and found it just had too many flaws when examined closely, and actually seemed equally as unbelievable as any religion.
I knew the religious exposure I had was wrong. I assumed everyone thinks you have to be good to get to heaven, and I knew everyone makes mistakes, some worse than others, so saw that as a flaw too.
I didn't believe in God but I didn't believe in evolution either, but back then evolution wasn't as big as it is now. I didn't really know what I thought about origins, I suppose I just didn't allow myself to think about it and kept busy instead.
I was reasonably happy despite having my fair share of life's problems, and was fiercely independent, so just trundled along for many years.
Eventually I stumbled across something new and decided to check it out for myself. It was the john 3:16 verse and I noticed it said "believe" and not "whosoever is a good person", or "whosoever gets the highest grades" or "whosoever is baby baptised". When I studied the word 'believe' I saw it meant to 'trust in' and 'rely upon' with 'full assurance'.
It was about believing God is who He says He is, the same as trusting that my GP is who he says he is. I look at his credentials on the door and on his ID badge and I believe they are true. With God I see his attributes are consistent within the bible, and the more I studied the bible the more I saw how each book compliments the other, despite being written by different people at different times. You couldn't easily achieve that even if you tried.
What I had misinterpreted to be contradictions were actually my failure to have studied it out and we all know what happens when you leap to conclusions without getting the right context. With study the right context was completely different.
I was studying for a degree at the time so the last thing I needed was to study something else at the same time, but when I saw the message of undeserved grace I was compelled to dig deeper. A free gift with no strings attached? How could anyone pretending to follow God miss that? It made more sense than any religion or belief, or even unbelief, than I had ever encountered.
And the fact that it's the oldest religious text, with some many archeological and prophetic accuracies - and none that are inaccurate - stood the bible apart from any other book I had read.
So I didn't make my choice over which God to follow by using my morals. It's true that God gives us an inbuilt conscience, but even with that we let ourselves down and even break our own morals. It is our lack of ability to follow morals that leads us to need a saviour in the first place.