Something like the little coin marker for pets that are for sale that shows up on pets that were caught as part of a festival color-scheme thing (or, after the system is implemented, were the child of two same-markered parents.)
Right now, there's really no obvious way to tell whether or not a Sylesti is or is not a festival pet - I've already seen a few arguments between people who bred nonfestival pets with someone's festival pets because they didn't know the color scheme...

When we redo the pet profile page they will be marked. They've been tracked in the database, so we know which ones are 'true' festival pets. So once the pet profile page is redone they will be marked.

It's in the works.

Krin has already implemented (a long time ago, like...March-ish, I think) a marker in the database that says whether a pet is from an event or not. It'll be shown later probably, when profile pet pages are redone. But yes, in the hidden database (that will be unhidden in time), it's marked. So don't lose the pet. :P
edit: darn, Fai beat me to it. xD
But the bottom part's still relevant. x:

One way to tell for sure, albeit tedious, is to compare the festival pet in question (or their bloodline) to an actual festival pet. You can tell it's a festival pet based on their:
1. Color Scheme (from eye color all the way to mutation color 3, it's a fixed template per event pet if you compare)
2. Captured status (somewhere along the lines, the parents (if offspring) or the pet itself should be of captured origin, not generated)
3. ID/Date of Birth (if the IDs or DoBs coincide with other pets caught within the event dates, then it should be from the event itself if it matches the first two characteristics above)

Yes, checking is tedious. But if you want "purebred" festival pets, then you'd have to be able to tell the original pet's template all the way to the "end result", and check on those parents. Of course, there's nothing wrong with actually breeding a purebred/real festival pet with something that isn't of kin blood. Just don't breed with it again and be wiser when choosing mates lol.

Oh, that's cool! Thank you for correcting me :)

(I'm excited to see what the Winterfest ones are going to look like...)