Does evanescent grace remove all assurance of salvation?

Evanescent grace, or vanishing Grace, is a phenomena addressed by Jesus in the Parable of the sower:

Matthew 13:18 through 23 KJV Hear ye therefore the parable of the sower. When any one heareth the word of the kingdom, and understandeth it not, then cometh the wicked one, and catcheth away that which was sown in his heart. This is he which received seed by the way side. But he that received the seed into stony places, the same is he that heareth the word, and anon with joy receiveth it;  Yet hath he not root in himself, but dureth for a while: for when tribulation or persecution ariseth because of the word, by and by he is offended. He also that received seed among the thorns is he that heareth the word; and the care of this world, and the deceitfulness of riches, choke the word, and he becometh unfruitful. But he that received seed into the good ground is he that heareth the word, and understandeth it; which also beareth fruit, and bringeth forth, some an hundredfold, some sixty, some thirty.

In this Parable Jesus is warning us that the Gospel will be received by people in one of four ways The first by the wayside is actually not assuming the Gospel. This is the incident where the listener is given the Gospel but instead of accepting it prefers to retain the pleasures of the World.

The next two are what you refer to as evanescent grace, In these two cases the hearer accepts the Gospel, but does not have enough fortitude to reject the things of the World or to withstand the ridicule Christians of the fourth kind are able to overcome.

To answer your question about how one can know that they are truly saved, we need to look at Jesus other revelations about gaining the Kingdom of God.

Luke 14:16 through 24 KJV Then said he unto him, A certain man made a great supper, and bade many: And sent his servant at supper time to say to them that were bidden, Come; for all things are now ready. And they all with one consent began to make excuse. The first said unto him, I have bought a piece of ground, and I must needs go and see it: I pray thee have me excused. And another said, I have bought five yoke of oxen, and I go to prove them: I pray thee have me excused. And another said, I have married a wife, and therefore I cannot come.  So that servant came, and shewed his lord these things. Then the master of the house being angry said to his servant, Go out quickly into the streets and lanes of the city, and bring in hither the poor, and the maimed, and the halt, and the blind. And the servant said, Lord, it is done as thou hast commanded, and yet there is room. And the lord said unto the servant, Go out into the highways and hedges, and compel them to come in, that my house may be filled.  For I say unto you, That none of those men which were bidden shall taste of my supper.

This is actually a warning; it tells us that we have a chance to accept salvation, but if we reject it others will be given our place in the Kingdom. But here is the good news:

Luke 15:1  Then drew near unto him all the publicans and sinners for to hear him. And the Pharisees and scribes murmured, saying, This man receiveth sinners, and eateth with them. And he spake this parable unto them, saying, what man of you, having an hundred sheep, if he lose one of them, doth not leave the ninety and nine in the wilderness, and go after that which is lost, until he find it? And when he hath found it, he layeth it on his shoulders, rejoicing. And when he cometh home, he calleth together his friends and neighbours, saying unto them, Rejoice with me; for I have found my sheep which was lost. I say unto you, that likewise joy shall be in heaven over one sinner that repenteth, more than over ninety and nine just persons, which need no repentance.

This tells us that Jesus will never stop offering us the Kingdom. 

 Luke 15:11 through 32  And he said, A certain man had two sons: And the younger of them said to his father, Father, give me the portion of goods that falleth to me. And he divided unto them his living. And not many days after the younger son gathered all together, and took his journey into a far country, and there wasted his substance with riotous living. And when he had spent all, there arose a mighty famine in that land; and he began to be in want. And he went and joined himself to a citizen of that country; and he sent him into his fields to feed swine. And he would fain have filled his belly with the husks that the swine did eat: and no man gave unto him. And when he came to himself, he said, How many hired servants of my father's have bread enough and to spare, and I perish with hunger! I will arise and go to my father, and will say unto him, Father, I have sinned against heaven, and before thee, And am no more worthy to be called thy son: make me as one of thy hired servants. And he arose, and came to his father. But when he was yet a great way off, his father saw him, and had compassion, and ran, and fell on his neck, and kissed him. And the son said unto him, Father, I have sinned against heaven, and in thy sight, and am no more worthy to be called thy son. But the father said to his servants, Bring forth the best robe, and put it on him; and put a ring on his hand, and shoes on his feet: And bring hither the fatted calf, and kill it; and let us eat, and be merry: For this my son was dead, and is alive again; he was lost, and is found. And they began to be merry. Now his elder son was in the field: and as he came and drew nigh to the house, he heard musick and dancing. And he called one of the servants, and asked what these things meant. And he said unto him, Thy brother is come; and thy father hath killed the fatted calf, because he hath received him safe and sound. And he was angry, and would not go in: therefore came his father out, and intreated him. And he answering said to his father, Lo, these many years do I serve thee, neither transgressed I at any time thy commandment: and yet thou never gavest me a kid, that I might make merry with my friends: But as soon as this thy son was come, which hath devoured thy living with harlots, thou hast killed for him the fatted calf.  And he said unto him, Son, thou art ever with me, and all that I have is thine.  It was meet that we should make merry, and be glad: for this thy brother was dead, and is alive again; and was lost, and is found.

If we dissect this Scripture what we come up with is that even though we were in the good graces of the Lord, and went astray he will welcome us back when we truly repent.

How can anyone know whether or not they are truly saved? the answer to that lies with the individual, only that person can know whether or not these Scriptures are fulfilled in them or not. And if the answer is yes then they can feel secure in their salvation. If the answer is no or maybe then some self re-examination is called for.

Hope this helps. 

Taking this teaching of Calvin into account, no man can logically know he is saved. He may feel and believe that he has saving grace, but he may be mistaken about that, or deceived. Therefore, he cannot know. He can believe, but he cannot know. It is a logical impossibility.