I have heard that at one point, Dominican brothers not yet having been ordained, wear a black scapular and then, once ordained, they wear a white scapular. Is this still the case? If not, how does one tell the difference between a Dominican brother who has been ordained and one who has not been?
One of our professed cooperator brothers, Paul Byrd, wrote a reflection on this topic on his blog in 2011, which can be found here: Dominican Cooperator Brother
In the article, he mentions: "After Vatican II, it was decided that all Dominican friars were to wear one common habit to emphasize the unity and equality of the friars."
In the first branch of the Dominican Order, the friars, there are two tracks an individual can follow: the clerical or cooperator brother track. The clerical brother pursues the priesthood, feeling a call first to the sacramental life. The cooperator brother feels a call to a consecrated life, but not necessarily as a sacramental minister.
The cooperator brother prior to Vatican II wore a black scapular, as seen with St. Martin De Porres. However after Vatican II, there was a call for many religious communities to return to their constitutions and reflect on the original spirituality (observances) that their founder/foundress wanted for their community. As you have seen, some Dominican communities (particularly the sisters) have either left the habit or modified it after this exhortation in Vatican II.
The friars felt that there needed to a renewal of Dominican observance, that there is a fraternal distinction as a brother in the order and the label of clerical or lay is secondary. As we would say, the friars see each other first as brothers. Thus to establish the primacy of the fraternal unity, the black scapular was modified to be white.
The way in which ordained and non-ordained brothers are distinguished is primarily during the celebration of Mass. The ordained brothers are seen in albs and stoles, while the non-ordained brothers are in their habits.