Your best bet is to obtain a puppy from a breeder that specializes in white. This way, there will be fewer suprises.
In regard to breeders that focus on other colors too, while the aim will be for white, if other colors are in the bloodline and skip one or more generations, there is always the chance that a white Pom will not remain pure white.
However, as stated above, any shadings of other colors , especially behind the ears, is a good sign that the Pom will NOT be white.
It is best if you also:
Ask to see the sire and dam (parents) of the puppy. It is best to see the parents in person. Photos can be extremely misleading (or in cases of unethical breeders, they can be touched up with PhotoShop).
If a breeder has the ability to produce whites, they certainly should be able to show you, in person, some of their white dogs.