I don't recall ever having seen a pied that didn't have a pure white belly. That's not to say that such a thing couldn't exist, but I haven't seen one. The pied morph is SO easily recognized, that a spot of color on the belly wouldn't rule one out, for sure.
Now...if you're looking at a "low white pied" from a questionable source, and it has color on the belly....the chances of it actually being a pied are pretty slim (and that's being generous). If you can post pictures here, we can tell you right away if it is pied or not. Even the lowest-white pieds are very obviously pieds.