Bah! It is an Illusion- a social lie enabled and encouraged by fairy tales and fantasies as well as romantic fools.
Love is the biological sensation caused by chemical reactions in the body to ensure procreation and survival of the species- it is temporary and situational.
You will know... Kinda like RobRich said- you wont be able to quite explain or understand it, but you will somehow know. If you are not sure, then you arent. Or, the above

.
Best of luck!