50% chance of a girl. The chance of a boy or a girl is very close to 50/50. It's the sperm that fertilizes the egg that determines the sex of your baby - a sperm with an x chromosome results in a girl, and a sperm with a y chromosome results in a boy. Timing intercourse in relation to ovulation has no effect on your chances, but there are no shortage of people or websites who say 'it worked for them'.
50% right. There are plenty of myths about predicting a babies sex and timing is one of them. A string test, how they are positioned, cravings, etc, are still all myths. They are right 50% of the time which is better than your lottery numbers, people seem to selectively forget the 50% of time they are wrong. An ultrasound will give reasonably accurate information in late pregnancy.