Or perhaps just do a cut to show a bit of blood, since Changelings own different blood colour.

@Background Pony #0F6E
He has occasionally; most obviosuly in his 6th, 9th, and war forms.

I this. One of his LOL moments. Many freak out instead of being as calm as he was with soldiers threatening.

@Background Pony #865D  
The doctor doesn’t shoot people

Royal guard: Nope.
 
Doctor: Right then. (runs off)

These aren’t Slitheen you’re dealing with, Doctor.

@Background Pony #8466  
Me: Ya wanna know why you’re a fake?  
Cadance: ?  
Me: Because of this…  
Gunshot  
GASP  
Celestia: Do you have any idea what you just did!?  
Me: Oh yes, because she’s a fake…  
Shining attends to Cadance  
Celestia: WHY DO YOU KEEP ON INSISTING THAT SHE IS NOT THE REAL CADANCE!?  
Me:……because there’s no mark on her……  
Celestia turns around to see no mark on Cadance  
Me: I aimed for one of her front legs…  
Flashback  
Gunshot  
Bullet flies  
Me: The bullet went on a trajectory for one of her vital arteries right next to the hoof, like the veins under a hand.  
Bullet flies through the leg and hits the wall on the other side  
Me: There’s only one species that I know that has more holes in them than swiss cheese…and I think I know who it is…  
Everypony looks puzzled while Fake Cadance get’s up all fine  
FCadance: So…you figured it out…no matter…  
Goes through a transformation while everypony gasps  
Me: Just as I thought, Chrysalis, queen of the Changelings. Codename: SWARM!

@Background Pony #B092  
That’s just wrong, Christopher was the best Doctor, and that was an excellent line.

worst doctor, worst line

Here’s an easy way to prove it
 
Shoot Candance and if she bleeds a different colour than it’s a changeling.
 
Celestia would understand, she’s seen Candance before hoof.

No, Doctor, I don’t think it will ever work.

*plain

Oh nice plane Doctor….