Propose to them lol but if you aren’t at that stage yet then do something REALLY REALLY REALLY special. Take the person out to something they love to do or somewhere they’ve always wanted to travel. It’ll show that you listen and really care

Wow, hard question. If you feel that way, the other person should know you well enough to know that you wouldn’t do that just for anyone, if you know what I mean.

I’m in a pickle! http://answers.yahoo.com/question/index?qid=20091017150915AAQPkU9

You show them.

~edit

Ok that doesn’t make sense. Sorry! I mean, I don’t think you can really show someone you love them unless you really do. Not to sound corny, but they’ll see it in your eyes. Like how you can tell that a mother loves her baby just by the way she looks at it?

In my opinion there’s nothing definitive you can do to prove that you love someone, it’s little things like coming round with some sweets and a film when they’re a bit down or buying little presents for them when they’re sick that really shows that you care.

Also, from personal experience all the grand gestures in the world can’t beat a really heartfelt ‘I love you’.

Well, there’s the grand gesture like in Just Like Heaven, where the guy builds Reese Witherspoon a gorgeous roofdeck garden.

Or a line, like from Roxanne, “I am, and will always be, the one who loved you without limits.”

Best bet is to show that you take her seriously as a person, not just a collection of highly interesting body parts.