In buds: a cross-section of a frozen bud shows a brown or black colouration up to the point of insertion on the branch. This can sometimes be difficult to observe because the secondary buds develop immediately and can mask the damage.
Frost damage at Château Latour in 2017
In shoots: a cross-section shows a brown or greyish phloem (a thin, usually green layer located under the bark through which sap circulates). The tips of more sensitive branches wither, turn brown and dry up.