Hello,
Is it possible to feel one's kidneys when lying face up on your back by placing your hand on your stomach, if you were really skinny? Would they be to the left and right maybe a bit up a bit of your navel?

Basically, no because as your kidneys are so delicate, they are encased in protective fat and are situated on the posterior abdominal wall, which would be behind your liver. If you lie on your back and put your hands on your belly, you would feel your liver first. Probably, the kidneys would lie below your belly button though.

The only other way to palpate your kidneys would be to lie on your side and press on the sides of your belly. If you have ever had a scan of your belly, then the probe goes right round the sides of your belly and towards your back to locate your kidneys.