I was walking in a crowd at college and a girl looked back at me what should i have done?

i was just walkin along goin to cl when this girl looked back at me, she was with her friend and it was obvious she was only interested in looking at me when she looked back... i know i should have talked to her but i didnt want to seem like a creeper just tappin her on the shoulder from behind while still walkin... so what would you have done if you were me in detail thx