That is not an easy question to answer, due to our fickle and immature emotions. Love is often confusing. It's easy for some to confuse love with lust, infatuation or emotional need. ONLY love will withstand the test of time. It is something that has to 'mature'. It either gets stronger over time or it wains.
It's only AFTER life's trials and tribulations that you can truly understand and appreciate that love is even present in your relationship. It will be tested. Only THEN, will you know.
In my opinion, you can tell by the way you feel each time you are around that person. You will go out of your way to make sure that the person you love is happy all the time and the person always makes you happy even when you are sad.
love its too much complex. when you think about a girl. when her everything seems very good to you. though she is super boring person, you pass very good time with her. all of these are sign of love. :)