Flirtation is a key sign that somebody likes you more than a friend. If you have a friend the same sex as you, you'll know that the activities of flirtation are much different than what you would do with just a regular friend or best friend. Normal friends don't tickle each other, wrestle, or feed each other things. Activities of flirtation often include physical contact of some sort. Flirtation is a key indicator that a person likes you beyond just friendship. So if they are flirting, this is a key first sign that you know they like you more than a friend.
Never rejecting you, a phone call, or event with you. This person will want to spend all the time they possibly can with you. Sure very important things can make exceptions to this rule, but in general they wish to spend hour upon hour with you. This time spent with you is another indicator that they really like you beyond friendship.
High interest put on your personal life, beliefs, family, & deep conversations. This person is trying to get to know you much deeper than just a normal friend may. Deep discussions on religion, children, life or philosophy. Things that wouldn't be discussed commonly with normal friends. Discussion about your family and/or an interest in your family as well.
Willingness to be there through times of sadness or trouble. To take time for you, and go the extra step to make you feel better. Yes, normal friends can do this as well, however, this will be accompanied with the person going the extra steps beyond what a friend would. They will be happy to help you and be there with open ears and arms at all times through your problems.
If all else fails, just ask them. Tell them its a normal thing for close friends of the opposite sex for something to exist beyond friendship. Just let them know that you are curious and just want to be honest with them. Be ready to answer the question back! Good Luck!