Get a soda can and throw a few coins in it.

When the pups are ready to go to their new home gather the ones you like and shake the can. It doesn't have to be shaken loudly, just enough to get a response from them. The one who is interested in the noise is the one you take and the one who is afraid of the noise is the one you leave.

This was how I chose my lab out of the group of pups.