That would be the good test of it. I took from the cabbage smell in Mrs. Figg's house that she brews Potions, but it could mean nothing.

I think that the Potions may only be usable by magical folk--that they interact not with normal physiology but with... again, whatever makes magic work.

I think that to be good Potions brewer, you would have to understand all the nuances of what makes the magical ingredients work and how they interact with the taker's magic. It's an intellectual discipline. A Muggle could theoretically master it, but the chances of a Muggle having such a complete knowledge of the magical world are pretty slim.