Historically, checking for circumcision was the surest way to determine who is a Jew or a Gentile. So does the plaque nailed to the cross by the Romans confirm that Jesus was circumcised?
We know that Jesus was circumcised because we are told that he was in the book of Luke:
Luke 2:21 KJV
And when eight days were accomplished for the circumcising of the child, his name was called JESUS, which was so named of the angel before he was conceived in the womb.
As Jesus said to the Sadducees:
Matthew 22:29 KJV
Jesus answered and said unto them, Ye do err, not knowing the scriptures, nor the power of God.
The Covenant requiring circumcision was with Abraham, and the Jews are the descendants of Israel, but they are only one of the Nations descended from Abraham, others include the descendants of Ishmael, of Esau, and that includes most of the Arabian peninsula nations.
It was no secret that he was Jewish he practiced and taught Judaism daily. Luke 2:21 says Jesus was circumcised on the eighth day. The plaque was a mockery because the people, the authorities, and the courts rejected his claim to be king. Romans did not take kindly to anyone who would challenge they're authority. The plaque itself was not proof he was circumcised, The proof would have been from luke 2:21, and from the fact that since he was obviously Jewish he would have been almost certainly been circumcised as it was required.