Aside from previous experience as a journalist, academic or professional writer, there’s no substitute for having written and edited many articles published on Wikipedia. A consultant should be happy to share their previous articles with you – if they claim they can’t because of confidentiality, then they are a “black hat” practitioner. Their client work on Wikipedia should always be disclosed already or they are in severe violation of core Wikipedia principles. You should also check out the expert’s user profile page to be sure they disclose they are a paid editor. If they don’t disclose, they’re a “black hat” editor. See above for an explanation as to why hiring a “black hat” editor is a bad idea.