Well, there are a few tell-tale signs... of which:
1. You have to understand that the State of Being "In Love", is Bliss - but, it's also insane, from the standpoint of rationality. In fact, Love makes people do extremely crazy things. Things you would never do based on hard Math or Logic. Example: Why love a pet? Pet's are expensive, don't help you pay rent or living expenses - BUT, we love them, right? What they give you back in "Love" is more than any material or physical thing you could Quanitfy.
2. Love is Boundless and Love is Eternal. True Love reaches even beyond space and time, sometimes even beyond death. My Aunt, who has been a Widow since 1982, since LOVES my Uncle... and she always will.
3. Young Love / Romance is probably the best. It gives every meaning to Life itself. EVERYONE always loves to see Young people in Love. True Love like this is always celebrated, because it's always genuine, extremely passionate, and usually gives us older people pause... in our own marriages, to recall what this was like.