Well I would imagine if you show a transfer of money from your accout to the building society or whatever then that is proof - however I presume you are doing this in case things go wrong in the future ... however much we hope they won't this site is full of people who end up wanting to split and one has paid much more towards the property than the other. However as you are getting married I don't think it would make much difference at the end of the day because you will both own all the house, unless you owned it as tenants in common instead of joint tenants which you may not have thought about. Good luck.