I would get someone to point them out on your boat.
They will be a dull silvery unpainted metal below the waterline but the shape is not important so this can vary
There is also sometimes one inside the engine
Here are photos to give you an idea .
They are easy to miss. For example the first photo
has 2 zincs one on the end of the prop and one between the prop and the sail drive leg. The second photo
has 3 one on each of the prop shafts and one on the rudder
The last one is three different sized engine
zincs, but you can only see the brass outer casing when its installed