a sole proprietor IS the owner, legally one and the same entity.
for example:
i own a sole proprietorship business. if i run off with your money, you can take me PERSONALLY to court. (i actually DO own a sole proprietorship). if the company goes into debt, it is held against me personally. the bank could foreclose on my personal property etc.
i also own a limited company, which means that the company and i are two separate identities. if my company goes into debt, the bank cannot go after me personally.
so, if you know who runs the business, chances are THAT is the sole proprietor. i honestly don't know if you can run a check on sole proprietorships. don't see how it would be different than for a ltd company.