It's also worth pointing out that Jesus said he was able to tell they were fake jews because if they were real jews then they would accept Jesus. Basically, all jews who don't accept Jesus are fake jews thus anyone who isn't a Christian but claims to be jewish is a fake jew, according to Jesus.

“If you were Abraham's children, you would be doing the works Abraham did, 40 but now you seek to kill me, a man who has told you the truth that I heard from God. This is not what Abraham did. 41 You are doing the works your father did.” They said to him, “We were not born of sexual immorality. We have one Father—even God.” 42 Jesus said to them, “If God were your Father, you would love me, for I came from God and I am here. I came not of my own accord, but he sent me. 43 Why do you not understand what I say? It is because you cannot bear to hear my word. 44 You are of your father the devil, and your will is to do your father's desires. He was a murderer from the beginning, and does not stand in the truth, because there is no truth in him. When he lies, he speaks out of his own character, for he is a liar and the father of lies. 45 But because I tell the truth, you do not believe me. 46 Which one of you convicts me of sin? If I tell the truth, why do you not believe me? 47 Whoever is of God hears the words of God. The reason why you do not hear them is that you are not of God.”

It's also worth pointing out that Jesus said he was able to tell they were fake jews because if they were real jews then they would accept Jesus. Basically, all jews who don't accept Jesus are fake jews thus anyone who isn't a Christian but claims to be jewish is a fake jew, according to Jesus.