You don't have to. They know it.
Often mistaken ...love is never proven...but identified. just like ..beauty is in the eye of the beholder.<br />
No matter how much we do for someone if he/she does not have the eye for it...it's not possible. You can look around and see it. There are so many people who are happy in their relationship not coz they are in a perfect relation but coz they think they are :)
First, you don't have to PROVE to someone that you love them. (If they are asking you to prove it, that's a red flag). If you really love someone, you'll be there for them when they need you. You'll decide you want to spend most of your time with your loved one, because that's what makes you happier than anything else.
I tell my girlfriend all the time I'll never be able to adequately find the words to describe how much I love her, words will never do me any justice. But I will spend the rest of my life showing her how much I love her. And I mean it, actions do speak louder than words.