Actually, you could get a silver Sharpie (Sharpies are non-toxic to humans, but you maybe could run this past the vet anyway), and draw a circle on the back of one and a + on the other, in the one small spot their tongues can’t reach. You’ll have to redraw it every couple of days. That’s better than having to turn them over and check their bellies every time you want to know which is which.
I thought they did that by kneading and clawing? I’m not sure that the teeny bit of dried nail polish would harm a kitten but I’m willing to be corrected. The nail polish thing isn’t uncommon amongst dog breeders, anyway, to identify the pups in a large litter of similar-looking puppies.
But really, assuming these are indoor kitties and they’re wearing well-made safe collars, there’s no reason they should come off. By the time the kittens are grown they’ll presumably be easier to tell apart.
I think I need a black kitten. Pick one and send it to me. Problem solved!
My cat rescue group uses a dot of nail polish on the tip of the ear to distinguish littermates, using different colors of nail polish when needed.
I wonder if they are identical twins!
If so, maybe one will be a lefty and the other a righty. Check which way they put on their belt. 
And we need pictures.
So… you’re saying all black kitties look alike?
[del]Racist[/del] Cat-ist!
Spayed, but no microchip.
Holy moley, if you insist on pix - don’t say I didn’t warn you: http://i1058.photobucket.com/albums/t412/g_grynberg/10885261_10154958057135716_3557760533197996710_n_zps7e414e53.jpg
Hopefully that works
They’re a lot easier to see in pictures than my black cat. Usually you can only see his eyes.
Nice job, black cats and dogs are hard to get good photos of!
Geez, I can tell them apart. One’s awake and one’s asleep. DUH!
That’s an excellent shot of two gorgeous kitties.
Odd. My cats have always done it on the sofa. Honestly, I have had cats most of my life. They claw at upright surfaces. I have never seen one chew on their nails. Where do you get the idea that they do it with their mouths?
Ear Tipping. effective, though some might consider it extreme.
I wondered that about a black cat I had years ago, and one of her littermates. You could tell all the kittens were littermates-- they were the product of a blue-point Himalayan who escaped one night, and some stray tom, and they all had the same eyes, but there was one long-haired B&W, one short-haired black kitten who was bigger than the others, both male, and then three short-haired females, one gray, and two black with tiny white spots on their bellies in the same place. I thought maybe the two black females were identical twins; they were a little bit smaller than the others, but just as healthy and well-developed. They definitely had hybrid vigor. Mine lived to be almost 18. I saw the litter a lot, because the mother belonged to one of my parents’ neighbors. You really couldn’t tell the two black females apart.
I wonder if paw dominance is inherited in cats the same way it is in humans? I think cats are a more even split in dominance-- something like a 2:3 ratio in favor of left, as opposed to humans, who are about 90% righties. Only about 10% of human twins are a left/right pair-- it just seems so remarkable in people who otherwise seem exactly alike, that you remember a pair if you meet them.
Two of our cats are brothers from the same litter and when they were kittens they were very difficult to tell apart from each other. They had different personalities though, and when my son switched collars on them one day to try to trick me, it only took me a few minutes to figure out what had happened. My son switched their collars on a few other occasions as well. The cats were close enough in appearance that I couldn’t tell which one was which just by looking at them, but once I watched their behaviors for a bit I could always tell which one was which.
Now they are older and one is bigger and fatter than the other, which makes it easy to tell them apart. We still often call them The Twins though.
My cat (who is very well trained not to claw the sofa, but we probably lucked out in that regard) chews on his nails all the time. He also has a scratching post, but he mostly uses his mouth to maintain his claws.
Different-colored collars (you can use a ferret collar if you can’t find a kitten collar).
A friend in college had four black cats. He told people he only had one. Made the parties more interesting.
Different coloured collars should work.
Or if you are feeling brave, you could trythis solution.
I’ve had several who chew at their nails; this isn’t unusual behavior at all. Maybe yours just do it when you don’t see, or you think they’re just washing their paws?
ABSOLUTELY NO TOXINS ON CLAWS!
Eventually, you will notice the pupils are slightly different.
Make sure their name both begin with a hard sound - and very different sounds - they hear only the first syllable.