No polite way to ask and this is not done, not. Unless, for example close family and a few friends know that the couple is having a hard time of it and they have a small wedding and friends and family throw a potluck reception for them. Gifts are not required at a wedding, a gift by definition is just that a gift, given freely and only if giver wants to. No registries, mention of money, financing honeymoon, nothing is mentioned in a wedding invite. It is an invitation to special friends and family to share your special day and not about gifts, but about being a part of your memorable day. Tacky to say the least. Honest answer.