The real issue is the absence of a BP genome map and the genetic tests such a map would allow for. Since there is no documented super form, it's assumed the homozygous and heterozygous forms are visually identical, and thus, a particular animal can only be suspected of being homozygous.
While you can suspect an animal bearing a dominant trait of being homozygous, the only "proving" you can do is when it's shown to be heterozygous by throwing even a single non-spider offspring.