Here's a new one I found out over the last year. While in the womb, both kidneys can fuse together making a horseshoe kidney. Turns out I've got one. With one tube going to my bladder. It hasn't effected my life so far. But yeah.

But to answer your questions. As others have stated it'll start with blood tests, then if something is found to be abnormal with them it'll progress to an ultrasound then an MRI. That's what happened with me anyway.