Flawless disguise.

IT’s so hard to tell. I just can’t do it.

Cadence is the real one shouting kill that bitch Chrysalis

Stop shouting, Chrysalis.

Spy check every pony!

BOIL WATER IN AN EGGSHELL!

If it’s good enough for Team Fortress 2, it’s good enough for Equestria!

Where’s pinkie?

That Cadance is a spy!

Throw them both out the window and watch how they fly. You should be able to identify Cadance’s flight style.

@Background Human  
“I… uh… what color was the frosting on the cake?”

Oh no! Which one do I shoot?!

@Star Cavalcade  
Welcome to the world of changelings. And they don’t even get the ass marks.

Quick! Ask them something only the real Cadance would know!

“PRINCESS IS SPY!”

…I can’t tell the difference o_o

Can you imagine being colorblind in a world full of identical looking ponies apart from their ass marks?
 
It would be the most awkard thing.

Sombra: Your lack of intelligence greatly disturbs me, Shining Armor.

“And now, she’s here to f*** us!”

For buck sake shining

This looks like a job for the Pyro.