1. I see you, you see me.
Always meet with him/her in person (either before you make the booking, or if you have already booked them following telephone conversations then definitely meet before the day).
Whilst we have Face Time and Skype nowadays, a wedding is very personal. A face to face meeting over drinks or dinner will help you both to get to know each other a bit better, and enables you to properly spell out your vision and the MC to understand better.
2. The right questions.
Ask the right questions to get the right information for e.g like how many weddings they have done, the type of weddings they have been involved in, the type of situations they have faced at weddings. This will help you gauge if they have the experience you require.
Also pay attention to the questions that they ask you. Are they just interested in getting another booking, or are they actually interested in understanding you, what you are about and what you are trying to achieve at your dream wedding.
3. Seeing is not always believing.
Watching a video clip of the MC telling a funny joke at another wedding is not necessarily important. Remember every wedding is different!!! (different couple, different set of guests, different day, different time).
What works at one wedding does not necessary work at another, what you need is some one who is adaptable to different audiences.
4. Funny or Fact
A person that can reel off lots of jokes one after the other does not necessarily make an MC. An MC is a person who can manage the emotions of an audience whilst monitoring all around him/her. They should be able to adapt to change within a wedding reception programme whilst keeping it on track.
Their people skills are equally as important as their ability to make you laugh. And on the subject of laughter, they should definitely be able to make you laugh!!
5. Quality over Quantity
In this case cheap is not always best. Costing is usually based on experience, I recommend that you go for experience all day, every day!