Hello My.  Some additional comments on your very valid question!  Professional headhunters (HHs) should know their domain very well.  What I mean by this is that HH's should be subject matter experts in their industry of practice, for example, IT&T, Consumer Markets, or Industrial, and be able to talk intelliginty about their practice.  On the client side, HHs should be able to convice you that they know their clients' business very well, and have a a good understanding of what their strategy is, what the management team is like, and what kind of talent they seek.  More importantly, you should feel your HH has a high degree of integrity and an appreciation of confidentiality.  As much as HH's ask you questions, you should feel free to ask as many question as you need to determine the professionalism of your HH and just how much they know and understand about their clients.