Canon 1066 establishes the norm for anyone who wishes to marry in the Catholic Church, whether Catholic nor not, whether a cradle Catholic or a convert, whether baptized or not, whether married before or not. It says, “Before marriage is celebrated, it must be evident that nothing stands in the way of its valid and licit celebration.”
Consequently each and every prior marriage that either party has celebrated, whether civil or religious, whether in the Catholic Church or not, must be examined to establish the freedom of the person to marry in the Catholic Church.
**This is true because a valid marriage sets up an impediment of divine law that prevents any subsequent marriage if that person from being valid, except in the case of a pauline privilege or a privilege of the faith or a dispensation from a non consumated marriage. But those processes as well as any process to determine the invalidity of marriage always require some examination based in Church law and the exercise of Church authority. **
Since this will require technical expertise and the exercise of authority in the Church, this matter is always best brought to the parish priest. Most have received sufficient training in Church law to know when they must rely on or seek guidance from the diocesan bishop. This is normally provided by the staff of his tribunal. But some cases will throw the best trained priests a curve since their training is to be pastors and not specialists in canon law.
That is the recommendation I make to the poster in the concrete circumstances.
I add the following only in response to the points discussed by Melford and Maritsa to illustrate just how technical things can become. I certainly applaud Maritsa and others who encourage the poster to speak with the parish priest though.
From a conceptual point of view, that Russian Orthodox are bound by their own Church to marry according to a sacred form in which an Orthodox priest confers sacred blessing on the couple. The Catholic Church accepts this into its own matrimonial law. Therefore, it is possible for a Catholic Church to declare the invalidity due to lack of sacred form in such a marriage between an Orthodox and a non Catholic. This will, of course, depend on more precise details of fact that require investigation.
But again, I say this only to highlight the importance of bringing such a circumstance to the parish priest, and he can be in touch with his diocesan tribunal.