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.
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!!
Love makes you feel a certain way, and really it has a power over you like nothing else. Really, it can be beautiful, it can be painful, and it can be everything in between, It can also be inspiration, motivation, but it can also be paralyzing and you'll know it when you feel it. Maybe not always at first, but you always feel it at some point or another.
Love makes you smile non stop. Your heart races at the thought of seeing that person. You think about them non stop. They make you want to be a better person. You would do absolutely anything for them. You trust them with everything. They are your best friend. And you just get this "home" feeling when you're with them.
-Thinking about the person every minute -Seeing the future with the person -Gets that dumb smile when you talk to the person -You REALLY want to talk to the person -You feel so happy and hype when you talk to them -Everything reminds you of the person -You would do ANYTHING for them