I love this concise checklist on detecting fake experts, with which my experience wholeheartedly agrees. The first one:

Imitators can’t answer questions at a deeper level. Specific knowledge is earned, not learned, so imitators don’t fully understand the ideas they’re talking about. Their knowledge is shallow. As a result, when you ask about details, first principles, or nonstandard cases, they don’t have good answers.

For more advice, see the original post.