Recognize when you are thinking about the other person outside of your regular interactions. If you find yourself wanting to share things you think about or witness during the day with the other person, then you have developed a link that goes beyond mere chemistry or friendship.
Keep your hands to yourself when you are around the other person and see how hard it is to do. If you always seat yourself near that person and constantly look for ways to touch, then you may be falling in love.
Laugh at the same jokes when no one else does and you may be in for a long-lasting love affair. Being able to get on the same wavelength with another person is a vital component of a love relationship.
Ache when you see that person with someone else and you may already be struck by cupid. When things do not go smoothly with someone that you love, it usually hurts. If you really don't care or feel disappointed, then there is no love lost there.
Think about that person all the time, even when you are with your current lover and you may have a problem that you will need to get honest about. If you can't get the other person out of your mind, no matter what you do, then you need to be honest with your current relationship and give yourself the space you need to explore your true love interest.