True Winter Whites and Campbells will each have certain colours and patterns, will be different size and will have differences in their 'type' (shape) as defined by show standards but pet shops will often mis-label what is basically a mix of the two species as either a Russian Campbells Dwarf or as a Winter White. You may also see names such as Djungarian or Sunfire but unless you've got a Pedigree which proves that the hamster is either a Campbells or a Winter White then these names mean very little and it's best to assume that your hamster is a Hybrid and a combination of the two.
Here's some information from the Midland Hamster Club about the two species...
Campbells:
Campbell's Russian Hamster
Winter White:
Winter White Russian Hamster