This is just saying, and can be used figuratively. Even for things, that cannot be seen at all. "if you drink this, you will feel more comfortable - I'll believe when I see it.”
As strange, as it may feel, it works this way. And I heard it used in this same sence even from people, who are totally blind (so it is impossible for them to see anything). The "when I see it" here means "when I could confirm it myself".
You could have this: "If you put this shell near your ear, you will hear the ocean - I'll believe when I see it.” for example.
There are many saying, that use "see" in figurative way and even for those, where it is not so figurative ("When I will see you again? - Tomorrow evening, just here." or "Let see, what happen if ...") - and a lot of blind people use such sayings as normal part of their speech - even when they will (de-facto) never see anything. But as the saing goes, they will witness the result in some way and it is easier to use common phrase with see, that to describe it in perfect scientific description.
Also it works in many other languages (I can confirm czech, slovak and russian, but I will bet, that also many others have such phrases and that even blind people are using it even if - literally taken - it is impossible for them)