Turning your hand to the side and in a fast, karate chop like motion chopping between the vagina. If done effectively the split check will result in a camel toe.
The split check is also a way of determining the gender of the truely androgynous.
The split check can also be performed between the butt cheeks.
"I ain't scared to split check a bitch!"
"My girlfriend split checked my ass, I kind of liked it..."
"After Alex split checked me, it hurt for days!"
"I couldn't tell if it was a guy or a girl, so I split checked it!"