Sign 1: It occured/occurs repeatedly and often.
If you've been or you are being picked on more than three times in rapid sequence, it's bullying.
Sign 2: You're being dragged down emotionally.
If someone is doing something to make you feel worse than you had been feeling, it's considered bullying.
Sign 3: You feel insecure.
If you have any insecurities whatsoever, these were probably caused by someone who's bullying you.