Not to sound trite but, we use the bible.
2 Timothy 2:19 Nevertheless the foundation of God standeth sure, having this seal, The Lord knoweth them that are his. And, Let every one that nameth the name of Christ depart from iniquity.
It seems to me that Paul makes a definitive statement here about who is and who is not worthy as a Christian, the very foundation of God given in scripture that most who call themselves 'Christian' would deny ... the question then is whether one has stopped sinning [not whether one has sinned, in the past, but if one has stopped hurting others with sin, braeking Jesus commands so that he is not one's lord if one does not obey the command to love, but sins against others]
It is not then a matter of whether one has sinned as all have sinned [excluding Jesus] ,but that Jesus showed us that one can live without sinning any more and all who follow him must do so else they continue to follow Satan as 1 John points out :-
1 John 3:8 He that committeth sin is of the devil; for the devil sinneth from the beginning. For this purpose the Son of God was manifested, that he might destroy the works of the devil.
the mark of a Christian is then unsurprisingly defined in scripture, one need not guess who is a follower of Christ, Jesus came to destroy sin, [not to perpetuate it under the guise of man-made religion of sinners]
again the scripture is clear that teh church of God si a congregation of saints, not of sinners , not that saints weren't once sinners for all have sinned, but that they departed from iniquity so that Jesus would accept them at his return... again the words of Jesus in scripture are clear are they not ? :-
Matthew 7:23 And then will I profess unto them, I never knew you: depart from me, ye that work iniquity.
Luke 13:27 But he shall say, I tell you, I know you not whence ye are; depart from me, all ye workers of iniquity.
Jesus states here incontrovertibly that he will not take anyne who has not stopped sinning against others in life...
again Paul states that the new covenant of grace is no basis for continuing to sin, God forbid! :-
Romans 6:1 What shall we say then?
Shall we continue in sin, that grace may abound?
2 God forbid.
How shall we, that are dead to sin, live any longer therein?
...
6 Knowing this, that our old man is crucified with him, that the body of sin might be destroyed, that henceforth
we should not serve sin.
7 For
he that is dead is freed from sin.
... clearly the scripture is at ods with the teachings of sinners, so one must decide wheteher to believe in divided sinners , countless denominations, sects, groups, individuals, or in the one integral truth of the saints and Jesus in scripture ...
we are then talking of nothing less than the foundation of God that sin is destroyed, not perpetuated, and indeed we can see the apostasy of the countless divided churches of sinners simply by reading the scripture of God ...