It's a common question asked at a birth, but when studying pbotos from the nineteenth century, it's often hard to tell whether a young child is a boy or girl.
In that time period, despite their sex, infants and toddlers all wore dresses.
The reason was simple. Until a child was potty trained, a dress or gown made diapering easier. Infants were dressed in long gowns and wore knit or crocheted pants called "Soakers" over their diapers. http://www.shasta.com/suesgoodco/newcivilian/kids/infants.htm
Toddlers of both sexes wore short dresses which enabled them to crawl and walk.
So, how to tell the difference in period photos? One clue is the trim. Although dressed alike, boys' fashions during the Victorian era had less frilly trim. Another difference is the hairstyle. Both boys and girls could wear their hair short or long, but boys always had their hair either parted on the side or wore bangs, while girls always had their hair, short or long, parted in the center. http://members.tripod.com/~histclo/dress.html
Of course, 21st century reenactor fathers have a hard time dealing with their young sons wearing dresses, even though it's historically accurate. But to 19th century fathers, this was a practical clothing style.