The way to tell is to look for sleeves. Dalmatics got 'em. Chasubles don’t. Traditional dalmatics may have two broad stripes (stirpes) and often in red. You’ll probably see that if you can find a holy card of Saint Lawrence, D-M. But the opportunity for fashion chaos is there, so most match the presbyteral chasubles.
The “new” General Instruction on the Roman Missal, n. 119 speaks of the sacred vestments: “b. For the deacon: the alb, the stole, and the dalmatic; the dalmatic may be omitted, however, either out of necessity or on account of a lesser degree of solemnity. . . .”
The recent instruction from the Congregation for Divine Worship and the Discipline of the Sacrament, Redemptionis Sacramentum, n. 125, is useful here: “The proper vestment of the Deacon is the dalmatic, to be worn over an alb and stole. In order that the beautiful tradition of the Church may be preserved, it is praiseworthy to refrain from exercising the option of omitting the dalmatic.”
The Episcopal Ceremonial mentions the possibility of a bishop wearing a dalmatic under his chasuble. I once teased our late and loved bishop about doing this when it was close to 100 degrees. He suggested I should wear two dalmatics.
In recent years, some dalmatics were made so the stole could be worn over them. The clearer sense now seems to be that the stole is to be worn under. So spotting a deacon may be less obvious if you can’t see the “diagonal” stole.
But the liturgical theology, if any is to be gained, is that the dalamatic, associated with the episcopacy at different times, is now consigned to the servants of the bishop, who are ordained, not to the priesthood, but to the service of the Church. Then too, even while they no longer wear dalmatics, priests do not cease being deacons because they are also ordained to the ministerial priesthood. Their service just takes a different direction.
Service of course is what it’s about, rather than liturgical haberdashery.
(Deacon) John M. Cameron, M.A. Theology, J.C.L.
Roman Catholic Diocese of Lansing
Lansing, Michigan