I don't quite think you can generalize to that degree - many snail breeders have kept their lines true to type. A reputable breeder can tell you the snail's history and their breeding practices. They will have clearly identified hybrids and kept them separate from pure snails. However, I think you're correct in that some snails in a LFS or snails from accidental breeders may have some Marisa blood. Anyway, we don't even have scientific proof that hybrids are reliably fertile. It's only been happening for three or four years, since spixies and marisas became readily available in the hobby. Even anecdotal evidence is rather slim at this point.
The best way to determine whether or not an individual spixi has marisa blood is to expose it to healthy plants. Hybrids should readily eat even healthy plants, while pure spixies will not touch them. I have also noticed that hybrid clutches are looser and the eggs are larger, much like the pure marisa clutches. The pure spixi clutches are easy to differentiate, with their small, opaque, tightly packed eggs.
The best way to determine whether or not an individual spixi has marisa blood is to expose it to healthy plants. Hybrids should readily eat even healthy plants, while pure spixies will not touch them. I have also noticed that hybrid clutches are looser and the eggs are larger, much like the pure marisa clutches. The pure spixi clutches are easy to differentiate, with their small, opaque, tightly packed eggs.
