Who is the original?

"Maybe we should determine who the original is." Once again both Urarakas spoke at the same time, surprising them. "Well obviously it's me but it would probably help to clarify..."

YaoMomo and Tsu simply looked in shock as the Urarakas started to glare at each other. The normally happy-go-lucky and bubbly girls were now getting incredibly annoyed with each other.

"Ochacco, calm down," Tsu said. "Let's just figure this out first."

"Okay." The Urarakas turned away from each other, still annoyed with each other.

"We need to run some tests of sorts." YaoMomo created a few bits of medical equipment, the supplies falling out of her bosom. "If we can find a difference between the two of you, we can use that to figure out who the original is and who is the clone."

That part was clear. The question was, which part did they need to test first?