Make them fight!

"This is odd," Kamek said, looking at them more and more. "They don't seem to have any differences from what I see. What about you?"

"Umm... well..." Bowser found his body drenched in sweat, unable to tell even the slightest difference between the Peaches. This was the woman he loved, the one he dreamed to marry so many times in the past, if no one could figure out the difference he should be able to. There must be, there must be some way he could tell...

"I-I-I-I think it's her," Bowser said, shakily pointing a finger at a random one of the Peaches.

Kamek, immediately seeing right through him, sighed.
"This is hopeless," Kamek said. "Just observing them is not going to do anything. Maybe if we have them compete in order to test their physical abilities we can finally find a difference between them!"

"Wait," Bowser said. "I'm trying to think."
It was hard to say after all, there seemed to be no difference between them. Nothing visible... was that the key to this situation?

"I got it," Bowser said. "We just need them to compete against each other in order to test their physical abilities! That way we can finally find a difference between them!"

"But that was my i-"

"I'm a genius! Why has no one thought of this before!?"

"Yes sire." Kamek just sighed and went along with it.

"So you heard me," Bowser said, turning to the Peaches. "The two of you need to fight! Beat each other up! Show us who the original truly is through your strength!"

"And why should we listen to you?" Peach stuck her tongue out at Bowser.

"What if I don't want to fight this fake me?" Peach crossed her arms and turned away.

"Fake you!? You realize you're the fake here!"

"What are you talking about? You're a fake!"

"Sire," Kamek said. "I don't think you needed to tell them, they'd probably fight regardless."