Hi tom8olvr,
It is very easy to do. Just take all the plump seeds and rinse them off the best you can. Then let them dry on a paper plate or paper towels.
Chances are it was grown in a big field of Hubbard Squash and will come true. I've grown out squash from the local squash farmer and had them come true. But there is always a chance it crossed with another type of squash.
In case you or anyone reading doesn't know, squash need insect pollination to make fruit unlike say tomatoes that pollinate themselves. So if a plant is grown in close proximity to anther variety, there is the possibility of cross pollination.
Remy