Ideally, you'd research the breed and the seller before buying one so that you'd know if the animal you were getting was purebred or not
If you got him/her from a reputable breeder, chances are it is a purebred. If from a shelter or a store, it might be mixed, but honestly unless you intend to use him/her for breeding (which you probably shouldn't if from a shelter/store since you won't know the lineage) is shouldn't be too much of an issue. Most rabbit breeds have similar care needs.