The branch on the holly shown in the photograph on the right was damage by the weight of snow that accumulated on it. Both the branch and its parent should be removed. Bark that has been knocked loose or torn off is unlikely to reunite with the cambium layer. Trim the ragged edges of the cambium layer and hope that the cambium layer can fill in the wound. Diseased wood is harder to recognize than dead or damaged wood.
Careful observation is the beginners best tool.
The infected stem or branch is going to look different from the rest of the plant. Wilted or dying leaves are an obvious indication of trouble. Some infections cause cankers to form at the point of infection. The area changes color and is sunken. In some cases the fruiting structures of fungi can be seen.