Most Helpful Guy
You just know it. I know that's not a good answer, but it's true.
That said, there are certain symptoms - at least that I know of for guys... For instance:
Can't wait to talk to or be with that person.
Would die for them.
Feel the blues when you cannot be with them.
Your world would turn upside down if they were no longer with you.1
Most Helpful Girl
First, when you choose to love him. When you can't see yourself with anyone else, even if you had the choice. When you try to get mad at him, but can't stay mad because it hurts you too much! When that fluttery feeling isn't quite there anymore, you choose to love him all that much more! When he's your BFF! When you're away from home and he's the first person you want to see when you get back! When he becomes the most important person in your life!!2