look at their Facebook page for reviews
and check that the reviews look legitimate - same for Google, are they positive reviews and are they real? (it's an unfortunate fact that some people pay for likes & reviews instead of letting them build naturally. How many likes or followers do they have? while a page can have too few likes, they can also have too many - yes, people actually pay for likes as well - just to make themslves look good.
look at their pricing and inclusions
- are there any hidden costs? travel etc. Price should not be the primary factor in your decision but it is important. Going for a cheap rate and getting a cheap product is never worth it.
Who turns up on the day
all too often people will double book the day or take any & every booking they can get and then farm the work out to sub-contractors. A month before your wedding they will be frantially advertising on private forums to find someone, anyone who can turn up on the day. Ask who turns up on the day? Do they use sub-contractors?
Look for experience
You may have a friend who knows someone who has filmed 3 weddings and they look fantastic - on a perfect day they might do a great job but it's knowing what to do when things go wrong that makes a professional, and that only comes with time. Don't risk your big day on a "weekend warrior" look for a production company that is full time. company XYZ may say they have filmed 60 weddings but that's only 2 years for the average videographer and it takes longer to gain the experience you really need.
Are they Local
Do you really want a company from interstate to film your wedding? If you absolutely LOVE their work and the price is right, maybe - just be sure it's them that turns up on the day. Lot's of companies will take any booking they can get and then try to find a local guy that will sub-contract to do the job - is that want you want? It really helps if your videographer is familiar with the area, knows the venues and other professionals you have booked for the day - they need to work together as a team to provide you with the best memories of your day.