In this case we are talking of whether a head covering is required or not required. If the law does require it then we need 2 or 3 witnesses to call it a sin.
Deuteronomy 19
15¶'One witness doth not rise against a man for any iniquity, and for any sin, in any sin which he sinneth; by the mouth of two witnesses, or by the mouth of three witnesses, is a thing established.
2nd Corinthians (Paul is quoting Deuteronomy)
1¶This third time do I come unto you; on the mouth of two witnesses or three shall every saying be established;
Deuteronomy 17. No death penalty is brought without 2 or 3 witnesses either
6By the mouth of two witnesses or of three witnesses is he who is dead put to death; he is not put to death by the mouth of one witness;
Heaven and earth are the witnesses of the Torah being upheld. Yeshua also answers the pharisees here in John.
John 8
13¶The Pharisees, therefore, said to him, 'Thou of thyself dost testify, thy testimony is not true;'
14¶Jesus answered and said to them, 'And if I testify of myself -- my testimony is true, because I have known whence I came, and whither I go, and ye -- ye have not known whence I come, or whither I go.
15'Ye according to the flesh do judge; I do not judge any one,
16and even if I do judge my judgment is true, because I am not alone, but I and the Father who sent me;
17and also in your law it hath been written, that the testimony of two men are true;
18I am one who is testifying of myself, and the Father who sent me doth testify of me.'
The scripture requires 2 or 3 witnesses before someone is guilty. If it is a sin to not cover your hair then that matter should be established by 2 or 3 witnesses. If it's not a sin, then each man can require and not require at his own choosing.