If we define communication as the perfect reproduction of your idea in another mind, that is impossible. Everything about us was created by the sum of all our heredity and environment and since those vary from person to person, not even the words we use will have identical meanings.
This presents us with a difficult task. Since our experiences and language do not completely overlap, even in the closest of relationships, we need to take special precautions during our communication attempts. Put another way: say it over and over again using different words and actions. After a period of time they will begin to understand what you mean when you say, "I love you."
On the other side, if you really want to hear "I love you" from that special person your temptation will be to apply your meaning of those words to what they have said. Take it easy and slow down. Determine over time what the word love means to them. If we would take the time to experience how the other person means in general, we would have a much better idea of what they actually saying.
This puts "whirlwind" relationships of all kinds in to a safe perspective. Move, but move with caution and many words and actions. That way you will have a better idea of who they are and they will have a better idea of who you are.