How can I tell?

I wouldn't know, either, especially without a pic. But I can think of one thing. How big are the paws in relation to the rest of the body. On large breeds, it's fairly common to have big paws, even at 6 months, inspiring many to say "he's gonna be big."

http://www.donnerbergrottweilers.com/rottweiler-teeth.html

that's the best I could find - I'm still looking. It says rottie teeth - but teeth are teeth =]

This is Snow:

Thanks for the link!

oooomgosh. she's adorable! looks like a spaniel/jrt to me =] what a cutieeee!

I started a thread on the Breed section

http://forum.dog.com/forums/t/80523.aspx

 Under four months, your pup will have all its "milk" teeth still.  These are small and finer looking than the teeth your adult dogs have.  If you compare the two, you'll see the difference easily.  Your pup is almost certainly under four months.  The question is how young he is - he looks very close to maybe ten weeks or less.  He might be as old as three months though - retrievers and boxers retain a neonate (rol-poly big head puppy) look until even after they start losing those teeth.

If he is older than four months, you'll see a mix of little and big teeth in his mouth.  By six months old all the adult teeth have replaced the primary teeth, under normal circumstances (sometimes you'll get a baby tooth that won't fall out).  Sometimes you'll get a dog that is on a somewhat shorter schedule.  Lynn had lost almost all her baby teeth by the time we got her, and her secondary set was well grown in, though she was only five months old.  We happened to know her birthdate, otherwise the vet would have put a much later age on her.  She still was sceptical and wanted to say she was eight months old.  Since Lynn's more than doubled in weight and grown nine inches since then, obviously her birthdate was correct.

If your pup has any herding breed or other working breed, paw size isn't going to be a good indicator of final size.  Almost all of my Border Collies have huge paws.  Gus' back paw wouldn't fit on a credit card, but he only weighs 45 pounds.  Wide paws seem to provide stability when running and leaping over uneven terrain.  I've noticed many hunting dogs tend to have the same oversized paws.