How do you know that Naruto love Sakura?

well later on in shippuden she does find out but due to the circumstances its not how naruto and sakura fans would want it you will understand after the pein arc where things really start to heat up

No, not yet. She knows Naruto stir things up all around her. She does feel warmth and love from Naruto but she also knows that Naruto is still immature and not ready for love