HOW COULD WE TELL IF A MACHINE HAS LEARNED?

We can do simply 2+2=4, but actually our BRAIN learned it by itself without teaching us all neurological (maybe millions of process) immensely complex processes involved in it to perceive and eventually just fool us with a sense that we learnt it. A human learns something, senses it naturally, applies it, tests it, stores it, and the amazing part is sometimes forgets it, sometime remembers it back. Our Brain blends observed and unobserved things incredibly, integrating observed (e.g., visual, audio, symbolic knowledge, etc.) and unobserved (e.g., intent, common-sense, feelings, etc) information to make learning happened in us. Now, Let me tell if a machine has learned something?