The fluttering claw movements that red eared sliders sometimes exhibit is most often a courtship ritual or "mating dance," though not exclusively. One turtle will face another head on and flutter or vibrate its front claws around the other turtle's head. It is most often males that exhibit this behavior, but not exclusively (so it can't be used to identify males and females). The performance of the courtship ritual does not necessarily mean mating will occur however, and sometimes it thought to be more of a dominance or territorial type of behavior.
To see what this fluttering activity looks like, have a look at this YouTube video (though the turtles here are both males and are likely engaged in some sort of social dominance display, as both are performing the behavior).
See Sexing Red Eared Sliders for more on telling males from females.