Picking a BMH pup is much the same as picking any other pup. What you are looking for is a healthy specimen with no deformities. At 8 - 10 weeks the pups should be weaned,feeding well and no signs of the skits. Eyes should be clear and no snuffles from noses. No scratching or smell from ears. Check teeth for any sign of over shot or under shot jaw. Don't just look at the one that takes your fancy have a look at a few.
8 week old pups can be sleepy things but if wide awake i would favour the pup that is aware of his/her surroundings and up to mischief.
I don't believe that you can pick a good working dog from a litter, you must rely on the pedigree. Most have their own way of choosing - the pup that comes to them etc. I like a dark coloured dog and when I picked my BMH I went for the dog pup with the darkest mask.