Conversation so far:
when does theory of mind begin in development
Between ages 4 and 5, children really start to think about others' thoughts and feelings, and this is when true theory of mind emerges.
what is theory of mind exactly
Theory of mind is the ability to attribute mental states to oneself, and to others, and to understand that others have mental states that are different from one's own.
do animals have theory of mind
The existence of theory of mind in animals is controversial.
what animals have been tested for theory of mind
Several taxa have been tested for theory of mind, including primates, birds and canines.

Question: how are the animals tested
Self-contained rewrite: how are animals tested for theory of mind