We are looking that the ways that the Magisterium can teach infallibly:
1. papal infallibility
2. solemn definitions of Ecumenical Councils
3. the ordinary and universal Magisterium
This universal Magisterium teaches infallibly when:
Although the individual bishops do not enjoy the prerogative of infallibility, they nevertheless proclaim Christ's doctrine infallibly whenever, even though dispersed through the world, but still maintaining the bond of communion among themselves and with the successor of Peter, and authentically teaching matters of faith and morals, they are in agreement on one position as definitively to be held. (Luman Gentium)
Now the Magisterium has not clarified this teaching the way that papal infallibility was clarified, so that there would be a set of criteria to discover if a teaching falls under the infallible universal Magisterium. However, the following points are reasonable and consistent with other teachings:
criteria for the universal Magisterium
1. the body of Bishops and the Pope (not the Pope alone, not the Bishops individually, not the Bishops without the Pope)
2. exercising their role as authentic teachers of the universal Church
3. teaching "one position definitively to be held"
4. on "matters of faith and morals"
5. the phrase "definitively to be held" implies that the teaching is a required belief, not a theological opinion.
Usually, it takes a period of time for an infallible teaching under the universal Magisterium to emerge. But notice that Vatican II did not mention a length of time. As long as the body of Bishops and the Pope are in agreement on one position, on a matter of faith or morals, definitively taught and definitively to be believed by all the faithful, then the teaching falls under the universal Magisterium.
Can anyone suggest some teachings which may fall under the universal Magisterium?