If one is married but not in the church, a civil marriage, and one in the party was previously married also not in the church but had a divorce without an annulment, is the second marriage considered by the church a valid sacrament or adultery? Does a blessing of the marriage require that an annulment be granted prior in order to make it a valid sacrament in the eyes of the church?
Can. 1060 Marriage possesses the favor of law; therefore, in a case of doubt, the validity of a marriage must be upheld until the contrary is proven.
If both parties of the civil marriage were not Catholic then their civil marriage is considered valid until the contrary is proven (i.e. an annulment).
If one of the parties to a marriage is a Catholic then they are obligated to follow canonical form and be married in a Catholic ceremony:
Can. 1059 Even if only one party is Catholic, the marriage of Catholics is governed not only by divine law but also by canon law, without prejudice to the competence of civil authority concerning the merely civil effects of the same marriage.
This means that all marriages that involve a Catholic in a wedding ceremony outside the Church or without the Church’s permission are invalid.
In the scenario you provide, if those civil marriages involve a Catholic then they are invalid marriages and do not need a full investigation to be proven invalid. All that’s needed to is to prove one party is Catholic and the marriage was outside the Church. At that point the two would be free to enter into a valid sacramental marriage through a valid Catholic wedding ceremony.