How To Identify That A Girl Is A Virgin ?

hello doctor , i am about to get married soon.i have never been with girls before. so i have lot of confusions.... i ask some of my married friends but still answers are confusing.......my question is ..... men gets erection when they think about sex..... does women have same thing like expansion of their part(vagina) when they think or started to do sex. how do i know the girl whom i am going to marry is virgin... my friend told me about his experience.... when he tried to make sex with his wife on first night it was not all success becos she felt pain and he had to take his penis out within a second after inserted for first time(one stroke) he found 3 drops of blood . so he didnt do sex for a week later he did, but his wife said she feel pain but no more blood.... what does that mean to you..does that mean is she virgin.. i am so nervous confused and excited about wedding n.b please only doctor answer this question please let me know ASAP................... THANK YOu dharmendar