Like nearly all Buddhist teachings, there are practices you can do to develop these faculties. For example, you can reflect that all beings have at one time or another been your mother, or your child; realizing this, how can you feel hatred towards any other being? It doesn't work for me, of course; I don't believe in rebirth.
> It doesn't work for me, of course; I don't believe in rebirth. Are you still able to generate loving-kindness in yourself? If so, then those teachings that don't work for you don't matter.
I suppose one could work from hatred; people generally know if they hate something. So if one has reduced the incidence of hatred, then perhaps you could say the technique worked. I used the technique (along with others) over a period of a couple of decades, during which I "suspended belief" concerning rebirth. But over a couple of decades, it's quite possible that my personality changed for other reasons - such as other techniques, or watching my children grow up, or simply maturing.