They hear the sound and will follow their littermates. Or if you're in the open they will look around for you, and then come charging in typical puppy fashion. It is not until they are older and range far out that they begin to have trouble navigating. The first step in checking pups for hearing problems is to be aware that the problem does exist and can show up in almost any litter, especially if there are a lot of white dogs in the line. Once you are aware of the problem, it becomes easy to check your pups.
Observation is the first step.
If you are at all suspicious then the pups need to be checked individually. A simple check is to hold a pup in your lap and then whistle softly. A pup that has two good ears will turn and look right at the source of the sound. A pup with one good ear will look up but will not be able to locate the source of the sound. If you are at all suspicious, the pup should be taken out where there is some cover.