I have bought several very yummy green/black pumpkins from the supermarket. I guess that they are kabocha (Japanese pumpkin) but I may be wrong (they don't have a species tag). Can I grow their seeds? I have thrown some seeds into my garden and they are growing leaves now. Will they bear pumpkin?

I've heard that some hybrid pumpkins can have infertile seeds. But since they already germinated and are growing leaves now, is it safe to assume that they can grow pumpkins in the future? I don't want to waste water/fertilizer/time on them and only later to discover that they can't bear fruits.

Thanks in advance!