If your parents have not already mentioned contributing money to your wedding, then there's no polite way to ask them if they plan on contributing. Because that implies that you are expecting a contribution, when in actuality they are not required to contribute.
So, if they haven't said anything about contributing, then keep your mouths shut. Plan the wedding with the assumption that you guys will be funding 100% of it. If the families approach you later with an offer to pay, treat it as a surprise bonus.
If they HAVE mentioned something about giving you money, then it's OK to contact them and say, "Hi Mom and Dad. I know you mentioned wanting to contribute something to our wedding. When's a good time for the four of us to get together and talk about what you want to contribute and what you'd like us to include in the plans."
That way you can figure out how much they're giving you, and what they expect in return for their money ... because, remember, money always comes with strings attached. Do not accept their money until you have spoken with them about what they expect it to cover (certain guests being invited, a church ceremony, specific flowers, whatever). And DO NOT make any solid plans until you have their money literally sitting in your hands ... because if they promise they'll pay $X, and then you go out and book $X worth of (for example) flowers that you couldn't otherwise afford on your own, and then later on they go back on their word to give you $X, you're left holding the bag for those flowers. So, if you are relying on someone else's money in order to pay for something, DO NOT book that thing until you receive that money.
If the parents have not mentioned giving you any money, but they are trying to push you into something you don't want or cannot afford, then it's OK to ask them if they plan to pay for it (assuming you're willing to accommodate their requests should they front the money - otherwise it's ok to just say NO and move on).
Example, if you guys set your budget and decide you can afford to accommodate 100 people, but then your parents insist that you invite 150 total people, you are welcome to say, "Sorry but our budget can't accommodate 150. If you really want these extra 50 people, it'll cost $X per head for food, seating, extra stationery and centerpieces. If you can get us a check and a list of their addresses by next Friday, we'll be happy to add them to our guest list. Otherwise we have to stick to the current list."
ETA: And, of course, it's worth noting that if your parents are planning to contibute to your wedding then they'll likely step up and mention it on their own. I very much doubt that, if they haven't yet said anything, they're just twiddling their thumbs and waiting for you to ask.
Asking them, "Do you plan to contribute?" puts them in an awkward position, where they look like diiiiiccckkks if they say no. Even if you preface it with, "It's OK if you don't!"