If it is a Belgian Warmblood, presumably it has registration papers you can ask to see. Or ask the lineage, sire and dam. IME, people with “real” warmbloods know all this about their horses, and take great interest in talking about it :). Ask the owners, see what kind of response you get. You could also later Google the names they give you and see if they exist in the Belgian Warmblood universe. I don’t think it’s a very common breed, certainly not in North America.
If they can’t name and brag about the registered sire, it probably isn’t a Belgian Warmblood.
That said, depending on the conformation that comes out of the mix, some people are happy with draft crosses at the lower levels of competition. But you shouldn’t be paying anything near warmblood prices for a draft cross, unless it has really proven itself in competitoin.