There is no substitute for first hand experience and learning from the experts, disease recognition workshops are a great way of learning to recognise the syptoms. Below are some reference photos which might help with diagnosis, there is lots more information on the FERA Beebase web site.
Firstly always know what healthy brood looks like. C shape, segmented and pearl white:
American Foul Brood (AFB)
Characteristics include greasy sunken closed cappings:
Notice all the scaling near the edge of the inside of the cell:
The tell-tale signs of the proboscis in one side with shrivelling:
Uneasy placing of collapsed brood:
Lack of segmentation:
The pupae do not develop or shed the last skin which locks in the pupae as it rots containing the spores.
The brood is pointing downwards but that is because the head does not develop:
Note in the middle that mites are present and pupae are weak and rotting:
The brood cells are cratered or raised:
Photos courtesy of the NBU and Ian R Homer firstname.lastname@example.org