Oo congrats on the new pup! I LOVE Bernese Mountain Dogs! Yes, do you have any pics? As Karen said, it would be pretty hard to not recognize those feet! I think it would be fairly easy to tell if he is close to purebred...as they are very LARGE dogs! (think black st. bernard lol........ok so they are not normally quite that big, but the first one I saw was!) If you think he is mixed with border collie, then he prob isn't a berner, because border collies are considerably smaller! Here's the size info on BMD from my book:
Male: 24-28 inches, 85-110 lbs
Female: 23-27 inches, 80-105 lbs