The anconeus muscle is a synergist of the triceps brachii and can give you elbow pain if you overwork it.
The repetitive extensions of your elbow, that are too much for you will be an individual matter and dependent on many factors like training status, diet and the state of your nervous system.
In general, the same activities that can overwork the triceps are relevant here too.
In sports, this may be exercises like the bench press, dips or swimming.
However, activities which do not seem damaging at the time may be too stressful for a particular individual.
This could be something like pressing down a piece of wood on a bench to stabilize it, so that your partner can cut it, or holding a heavy drill in order to place a bolt in rock or dense wood.
Finding the anconeus is not too difficult.
For this one, use your fingers for massage. A massage ball might already be too big and thus will not get on the muscle properly but your fingers will be just fine.