Basically the title. If the coach is certified with a tag, can we be sure that he is indeed who she/he claims to be, i.e. that he held that exact position at an MBB firm, etc.?
Not necessarily - some coaches have rather vague information - ie no surname, worked at multiple firms, etc, so finding them on LinkedIn is rather hard, especially if they are not working currently.
On top of what Ian says - look out for contributions in the Q&A section and you will see those experts like Ian who gives solid advice.
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Thanks Robert! :)