For most turtle species it is tough to sex babies, but as they get older, it gets easier. The most common indicators are:
Male: Long and thin tail, concave plastron (concave- incurved, plastron- belly shell)
Females: Short and wide tail, flat or convex (outcurved) plastron.
Some species exhibit other characteristics:
- Ornate box turtle's- male eyes tend to be red/orange, females are brown.
- Red-ear sliders- male necks tend to be more colorful
- Many pond turtle species- males have very long front claws
PS: You don't usually 'probe' a turtle for sexing.