Hashgachot follow up Q

Question

Dear Rabbi, In your response to Sam you state that an fully observant Jew who owns a restaurant does not need a Mashgiach - and then go on to qualify that the person must be FULLY observant. Isn't the halacha of Eid Echad Ne'man B'isurin enough? And, if it is, what kind of checking do we need to do to determine who is an Eid who can be Ne'eman? This should effect not only which restaurants we eat in, but who's house we eat in! Thank you!