I thought you looked for a dent in the plastron, flat belly=girl indentation=boy.
Shanti's right about the long tail thing, but I think you'd need a male and a female so you could compare tail lengths?
A quick google search said no hemipenes.
They have a penis and sexing varies significantly from species to species, although certain taxonomic subgroups are often somewhat alike across species. Do not rely on plastral concavity versus convexity alone. There are many exceptional individuals.
Male sliders, painteds, maps, and some others have longer foreclaws (usually, but not always), and their females are often larger, but not always. Most of the other turtle species do not exhibit elongated foreclaws because fanning is not a part of every species' courtship ritual.