My first suggestion when booking a videographer for your wedding, is to watch as many films as you can. The absolute best way to get a feel of what a videographer has to offer, is to watch the final product. After you find a videographer with films that give you all the feels, here's a list of dos & don'ts to ask them before you book:
DO ask: What makes you passionate about filming weddings?
DON'T ask: Did you go to school for this?
It's always important to know where someone's motivation comes from. If they are creating work they love and gives them life, it always shines through. It not only affects how deeply the film connects to a couple emotionally, but it also affects how they work with you and other vendors. Passion comes from a place in you heart, not where or how long you were educated for. When it comes to a creative trade, you could learn about it for years, but without a deep love for what you're creating...none of that matters.
DO ask: How do you go about capturing the day?
DON'T ask: What type of camera do you use?
Everyone's filming process is different, but it's important to know if their process works for you. If you are someone who likes a more candid feel, you are probably looking for a videographer that is quietly working behind the scenes (when you forget someone's filming, it's easier to capture those raw moments). If you are someone that loves to be directed with a more styled approach, you probably want someone who gives direction side-by-side with the photographer. Again, the best way to know the quality of a videographers footage, is to watch it! It doesn't matter the price/brand of the camera. If the final outcome is beautiful, that's what counts.
DO ask: What is your editing process?
DON'T ask: How many weddings have you shot?
The editing process after your wedding can be even more important than the actual wedding day itself. Post-production is when all the magic happens! It's important to know things like: do they edit with the flow of the music, do they follow the same editing structure for every wedding, what happens if you want to make a change after the film is edited, etc. Experience is important, but the best way to judge is based on their portfolio of work, and how well they communicate with you. Just because someone's filmed over 100 weddings, doesn't mean the outcome will be better than someone who's only filmed 20.
Hope that helps!