Okay let's clear one thing up right off the bat: No one is saved or damned until the moment of death. Up until then no one knows for certain.
Second you are forming in the womb for nine months before your actual birth. You are not half out of the womb are you?
So you cant know if you have it ... and you know for sure that you cant know ... that is what you do know for sure, that you cant know ...
1 John 5:11-13
11 And the witness is this, that God has given us eternal life, and this life is in His Son.
12 He who has the Son has the life; he who does not have the Son of God does not have the life.
13 These things I have written to you who believe in the name of the Son of God, in order that you may know that you have eternal life.
Present tense all around ...
Doesnt say anything about "He who has the Son will find out if he has the life when he passes from this one" ... or ... "Upon death we find out if we have the life" ...
Seems like there are only two possibilities here ... either the person who is saved and has no assurance is not a mature believer ... or they are not a believer at all.