A sure fire sign that a man is flirting with you is when he puts his hand in the small of your back and gently pulls you towards him. He also stay closes to you when you're out together.
Men are seriously flirting when they keep eye contact with you when you're talking. They give you their undivided attention and remember little things you tell them for future conversations. He finds ways to see you again or eagerly offers to fix something for you. If you're out with friends and he opts to stay with you if the others leave, he's definitely flirting with you.