Dominus Iesus (English: The Lord Jesus) is a declaration by the Congregation for the Doctrine of the Faith (known as the "Holy Office"), approved in a Plenary meeting of the Congregation and signed by its then Prefect, Joseph Cardinal Ratzinger, later Pope Benedict XVI, and of its then Secretary, Archbishop Tarcisio Cardinal Bertone, later Cardinal Secretary of State. The declaration was approved by Pope John Paul II and was published on August 6, 2000. It is subtitled "On the Unicity and Salvific Universality of Jesus Christ and the Church". It is most widely known for its elaboration of the Catholic dogma that the Catholic Church is the sole true Church of Christ.
IV. UNICITY AND UNITY OF THE CHURCH
17. Therefore, there exists a single Church of Christ, which subsists in the Catholic Church, governed by the Successor of Peter and by the Bishops in communion with him.58 The Churches which, while not existing in perfect communion with the Catholic Church, remain united to her by means of the closest bonds, that is, by apostolic succession and a valid Eucharist, are true particular Churches.59 Therefore, the Church of Christ is present and operative also in these Churches, even though they lack full communion with the Catholic Church, since they do not accept the Catholic doctrine of the Primacy, which, according to the will of God, the Bishop of Rome objectively has and exercises over the entire Church.60
On the other hand, the ecclesial communities which have not preserved the valid Episcopate and the genuine and integral substance of the Eucharistic mystery,61 are not Churches in the proper sense; however, those who are baptized in these communities are, by Baptism, incorporated in Christ and thus are in a certain communion, albeit imperfect, with the Church.62 Baptism in fact tends per se toward the full development of life in Christ, through the integral profession of faith, the Eucharist, and full communion in the Church.63
This is a very simple question and a more simple answer to respond to.
In this Declaration, the Roman Catholic Church recognizes the validity of Orders and Sacraments of Old Catholic denominations from catholic bishop to catholic bishop in a unbroken apostolic line:
"The Churches which, while not existing in perfect communion with the [Roman] Catholic Church, remain united to her by means of the closest bonds, that is, by apostolic succession and a valid Eucharist, are true particular Churches."
"Therefore, these separated Churches and communities as such ... have by no means been deprived of significance and importance in the mystery of salvation. For the Spirit of Christ has not refrained from using them as means of salvation which derive their efficacy from the very fullness of grace and truth entrusted to the Catholic Church."
Answer: by this statement any catholic bishop in an unbroken apostolic line from the original Roman Catholic Bishop is a true Catholic Bishop, but we must remember to stay as close to the original source as much as possible.
But then if you read the statement further it refers to those of the Anglican communion, Methodists etc are not true churches but communities with invalid orders taking in to consideration the Papal bulls of Apostolicae Curae on the Nullity of Anglican Orders, Pope Leo XIII - 1896, reaffirmed by Pope John Paul II and furthermore by Pope Emeritus Benedict XVI.