Modern Christians like to default to the "only Jesus knows" response to who is saved but Scripture doesn't leave us wondering. It is very clear about who is and isn't a Christian and how one can tell the difference. The apostle Paul wrote,
2 Corinthians 13:5
5 Examine yourselves as to whether you are in the faith. Test yourselves. Do you not know yourselves, that Jesus Christ is in you?--unless indeed you are disqualified.
Why would Paul write such a thing if it is impossible to know if one is actually saved or not? Why bother to test oneself if only Jesus really knows who is saved? The apostle John seemed to think as Paul did; that one can know if one is saved or not.
1 John 5:11-13
11 And this is the testimony: that God has given us eternal life, and this life is in His Son.
12 He who has the Son has life; he who does not have the Son of God does not have life.
13 These things I have written to you who believe in the name of the Son of God, that you may know that you have eternal life, and that you may continue to believe in the name of the Son of God.
If one can test oneself and see if one is truly saved, if one can know one is really born-again, then it seems to me it is possible to make that assessment of others. One's salvation cannot be kept a secret. If one is truly born-again, it will inevitably show up in how one lives and the attitudes one adopts. The writers of the New Testament certainly thought this:
Matthew 7:17-20
17 Even so, every good tree bears good fruit, but a bad tree bears bad fruit.
18 A good tree cannot bear bad fruit, nor can a bad tree bear good fruit.
19 Every tree that does not bear good fruit is cut down and thrown into the fire.
20 Therefore by their fruits you will know them.
Romans 6:22
22 But now having been set free from sin, and having become slaves of God, you have your fruit to holiness, and the end, everlasting life.
Galatians 5:22-23
22 But the fruit of the Spirit is love, joy, peace, longsuffering, kindness, goodness, faithfulness,
23gentleness, self-control. Against such there is no law.
Ephesians 5:8-10
8 For you were once darkness, but now you are light in the Lord. Walk as children of light
9 (for the fruit of the Spirit is in all goodness, righteousness, and truth),
10 finding out what is acceptable to the Lord.
1 John 3:14
14 We know that we have passed from death to life, because we love the brethren. He who does not love his brother abides in death.
Jeremiah 15:16
16 Your words were found, and I ate them, And Your word was to me the joy and rejoicing of my heart; For I am called by Your name, O Lord God of hosts.
Romans 8:16
16 The Spirit Himself bears witness with our spirit that we are children of God...
1 John 2:3-6
3 Now by this we know that we know Him, if we keep His commandments.
4 He who says, "I know Him," and does not keep His commandments, is a liar, and the truth is not in him.
5 But whoever keeps His word, truly the love of God is perfected in him. By this we know that we are in Him.
6 He who says he abides in Him ought himself also to walk just as He walked.
Selah.