How do you know when someone's vegan?

Don't worry, you'll be able to tell by the small crowd gathered around them asking them inane questions like:

"But what if the animal consents though?"

"If you were stranded on a deserted island..."

"If everyone went vegan then wouldn't that make farm animals go extinct?"

"What about if I tell you that for every piece of meat that you don't eat, I'll eat two?"

etc. etc.