Besides observations of behaviour, there are also neurological evidence (e.g. Do they have structures functionally similar to those important/responsible for emotions in humans and are they important/responsible for similar behaviour in these animals? Are they actually evolutionarily preserved structures?), and evolutionary/adaptive arguments, although these ultimately tie back to behaviour in some way, but sometimes specifically human behaviour, not the animals’ behaviour, although both together could strengthen the argument.
Besides observations of behaviour, there are also neurological evidence (e.g. Do they have structures functionally similar to those important/responsible for emotions in humans and are they important/responsible for similar behaviour in these animals? Are they actually evolutionarily preserved structures?), and evolutionary/adaptive arguments, although these ultimately tie back to behaviour in some way, but sometimes specifically human behaviour, not the animals’ behaviour, although both together could strengthen the argument.