I honestly don't know I can tell you how "you" know you're saved, but I can testify about how I knew I was saved and pray the concepts are used by His Spirit to relay the truth. One thing I have learned since being saved is it is a very personal thing and can happen in completely different ways for different people.
An example is my rebirth was a miraculous, overnight, completely changed from the inside out, in total awe of what happen completely dying of hunger for everything God everything Jesus! Now I hit my knees after already having thought I was a Christian, I went up front, said the prayer, was dunked in water and told I was saved now. After 5 years or so of "liking the idea of Jesus" but never having any kind of change of any kind I was part of the club, then tragedy struck. I was in a motorcycle wreak and loss all function in my dominate right arm. This crushed me. Over 2 years of the world crushing me, of wanting to die so bad I though about killing myself literally all day every day. Without hope, the doctors couldn't help me, science couldn't help me, the lawyers couldn't help me, and deceived into thinking I already knew Jesus and was saved, at that point in my head even God couldn't help me, or wasn't any help anyway.
After two years it all came to a head and I broke down and hit my knees. If I ever believed in God before truly, I didn't now. I hit my knees in defeat and seeing what a futile joke "my way" in life was, and where it lead. I hit my knees and quit only to be picked up by Jesus. Early the next day I made a comment to the receptionist that "I feel like I have an umbilical cord to the Holy Spirit", which was really a weird thing to say, but I went about my day. It was lunch time a bit later when it hit me. It was so earth shaking to me I almost couldn't speak and it hit me like a ton of bricks. "I HAVEN'T THOUGHT ABOUT KILLING MYSELF ALL DAY!!!!!!!!!!!!!!", And it was right then in that moment I KNEW that whatever this was, whatever happen was God. Even bigger and more important I KNEW Jesus was His Son.
From there He lead me strait to John and by the time I hit chapter 3 I was back at the receptionist desk proclaiming in astonished excitement "I think I was born again!". This was how I knew I was saved, and He's been with me every day since, to His glory.
This is how "I" knew I was saved. For a bit of contrast my wife said this took place for her in a room alone with the bible seeking Him out in truth and honesty. Amazing, and I know my testimony sounds more flashy and entertaining, but standing in front of God, face with these two resamples, I'd rather be able to have hers. In a sense, and just as an example.
Another testimony I've heard is just having served Him as long as they could remember. Raised in Christian homes and feeling called at a young age I know 2 people with this testimony. Now saying this I do not want to give the impression I'm talking about being "born Christian" or anything like that, because I do believe there comes a time when a transformation must take place, Jesus tells us we must be born again, so I have no idea when this took place in their lives, and they didn't pinpoint a time this happen, but this is another way we might be born again.
I'm sorry I can't tell you more than these testimonies and examples, but the one thing we all do share is the spirit. I mean the Holy Spirit too of course but what I mean as well is our spirit. When we are born we are born into this world from Adam into sin, born with a dead spirit. Inside us this dead spirit is what we feel is wrong inside of us, this is what we try to repair through the flesh, and can never know it won't work, because it's not the problem. The thing is that without God directly telling us the problem we could never even know what it is and why the flesh can never satisfy. But how we know we are saved, and what being born again is, is our spirit resurrected and reconciled to His, as we were created to be. Made whole.
so He can never say to you "I never knew you". 