You can know what people believe inside--by what they do on the outside.

Matt. 7:13-20 "Enter ye in at the strait gate: for wide is the gate, and broad is the way, that leadeth to destruction, and many there be which go in thereat: Because strait is the gate, and narrow is the way, which leadeth unto life, and few there be that find it. Beware of false prophets, which come to you in sheep's clothing, but inwardly they are ravening wolves. Ye shall know them by their fruits. Do men gather grapes of thorns, or figs of thistles? Even so every good tree bringeth forth good fruit; but a corrupt tree bringeth forth evil fruit. A good tree cannot bring forth evil fruit, neither can a corrupt tree bring forth good fruit. Every tree that bringeth not forth good fruit is hewn down, and cast into the fire. Wherefore by their fruits ye shall know them."

Matt. 12:34  "O generation of vipers, how can ye, being evil, speak good things? for out of the abundance of the heart the mouth speaketh.

If a person lives a certain lifestyle, it can be known, according to scripture, that the person is bound for hell.

I Cor. 6:9-10 "Know ye not that the unrighteous shall not inherit the kingdom of God? Be not deceived: neither fornicators, nor idolaters, nor adulterers, nor effeminate, nor abusers of themselves with mankind, Nor thieves, nor covetous, nor drunkards, nor revilers, nor extortioners, shall inherit the kingdom of God."

Matt. 5:19-21 "Now the works of the flesh are manifest, which are these; Adultery, fornication, uncleanness, lasciviousness, Idolatry, witchcraft, hatred, variance, emulations, wrath, strife, seditions, heresies, Envyings, murders, drunkenness, revellings, and such like: of the which I tell you before, as I have also told you in time past, that they which do such things shall not inherit the kingdom of God."

The two passages directly above name lifestyles of people who are not bound for heaven--such people as fornicators, adulterers, homosexuals (abusers of themselves with mankind), drunkards, etc.  Everyone on the face of the earth is a sinner and capable of getting drunk, lying, stealing, and so on--even Christians can make these mistakes. However, if an individual continually returns to these sins and remains in these lifestyles then it can be said, without doubt, that such a person is bound for hell unless they turn to Christ and accept him as their savior.

Will a Christian ever return to the sins of the past?

Sure, maybe once and a while, but it won't be near as often, and a feeling of guilt and conviction will come with each mistake.

2 Corinthians 5:17 "Therefore if any man be in Christ, he is a new creature: old things are passed away; behold, all things are become new."

Please note: Some people are good at wearing a fake spiritual exterior, while being corrupt on the inside.

Matt.7:15 "Beware of false prophets, which come to you in sheep's clothing, but inwardly they are ravening wolves."

How do you receive salvation, does it have to do with how good a person is?

Although you can know a person is not saved by what action they take, salvation--a home in heaven--is not obtained by the works a person does at all. Salvation is the result of a person's belief and faith in Jesus Christ and how he died to pay the penalty for their sins.

Eph. 2:8-9 "For by grace are ye saved through faith; and that not of yourselves,: it is the gift of God: Not of works, let any man should boast."

Rom. 3:23 "For all have sinned and come short of the glory of God."

John 3:16 "For God so loved the world, that he gave his only begotten Son, that whosoever believeth in him should not perish, but have everlasting life."

John 3:15 "That whosoever believeth in Him should not perish, but have eternal life."

Rom. 10:13 "For whosoever shall call upon the name of the Lord shall be saved."

I John 5:13 "These things have I written unto you that believe on the name of the Son of God; that ye may know that ye have eternal life, and that ye may believe on the name of the Son of God."