Not sure what exactly you mean by "know". But yes, a girl will know when she has semen inside her, because it feels wet and tends to ooze back out (however, pre-ejaculate will probably go unnoticed). She can't feel the specific sperm as they travel up the vagina and into the uterus.
a majority of semen is actually just other stuff and fluids. Once the sperm start to travel along the cervical mucus to their destination, a lot of the extra fluid may leak back out over the course of several hours. That doesn't mean the sperm is coming back out though, just the semen.