You simply can't place a value on an experienced professional on a wedding. Your photographer needs to know what will happen before it does, anticipate moments, be flexible on the spot, know how to help you schedule your day-of time-line, and be able to conquer any lighting (or weather!) condition. Here's a few things you can ask.
Have you ever done a wedding like mine? It doesn't matter if your photographer has shot at your planned venue. What matters is if they've photographed your type of wedding like religious ceremonies, traditional, high-end or micro. This doesn't have to be a deciding factor, but it helps if they know what shots to take during the day.
What happens when_____? You fill in the blank. Ask the photographer a crazy question to see if they can think on their toes.
Do you put together family photos? Your photographer should discuss with you what family shots you want before the wedding and make suggestions as to what shots to get based on your family members and the amount of time you have for pictures. This shouldn't be your job the day of! I make sure to go over this with all of my brides well before the wedding.
Do you have a videographer/cinematographer you work with regularly? You'll want your visual artists to be in-sync the day of so that they don't get in each other's way, but instead better your final product by working together.