Okay how about this, "God will let you know and that is the only way we can KNOW." I can't tell anyone else how to KNOW they are saved, all I can do is share with them how I KNOW that I am saved and pray the Spirit opens their eyes to it.
Now how do I "KNOW" I'm saved? Well I've shared that on this site to the point I almost don't like to just so people don't get tired of seeing it, but it was through the hardest trial of my life. It was after 2 years of wanting to kill myself every day and thinking about doing it at least every 5 minutes of everyday that whole 2 years. It was after a motorcycle wreak that wasn't my fault took all function from my right (and dominate) arm, there was no way I, modern medicine, science, or this world could fix it, and me already thinking I was a Christian (the one that would have heard "depart from Me I never knew you") so I had nowhere to turn. I got no compensation for it at all, no benefit at all, yet could not leave my 2 boys with an "my dad killed himself" story, and knew know one would love them like their father would no matter how bad I wanted to die.
I came to the end of myself, I knew there was nothing I could do to help myself and I threw my hands up and quit. That's what I did the day God saved me (Sept. 29th 2013), I honestly didn't even want to believe there was a God anymore, if I every truly had, at this point. I hit my knees just knowing I couldn't do it anymore, and that's what I said, "I can't do this anymore, I can't do this anymore, I can't do this anymore, I tried, world you win, and I QUIT!!!" That was it, I didn't realize it then but the "old man" died right there on the floor that day. The next day I woke up new, I didn't even realize until lunch that I hadn't thought about killing myself all day, I also realized right then that I had felt better all day too. At that point I had no clue what had taken place but I KNEW two things, I knew it was from God, and I KNEW Jesus was His Son.
I started really thinking and then for the first time ever picked up the bible then started reading it for myself. Then I thought after reading a few things "I think I was born again", then kept on reading and reading. Within an hour or so I was just plain awestruck, It's all real? I't like for real really real? OH God thank you it's like for real really actually real. I think I dragged my jaw behind me for a month after this, after my own "road to Damascus" experience. That was how God assured me I was saved, and that is how I KNOW beyond a shadow of a doubt that I am saved. I can't tell you how you know you're saved, or anyone else either, that's between the person and God, each one of us our different, but the way you KNOW you're saved can only come from God. Hope that's specific enough for you.