For SEO, ask the following questions mentioned by Rand Fishkin (he really knows his stuff)
For Adwords, use these questions from Wordstream since they know Paid Search inside out!
For Social Media, David really knows his stuff about that. He created a list of questions you should ask. http://www.therisetothetop.com/davids-blog/key-questions-hire-social-media-marketing-consultant/
Hope this helps!