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I`d like just to ask a question... mm... if the one year old red ear slider (Earl) (his front legs extended outwards) can shake (flutter) it`s nails very quickly with in 6 months turtle face, so can it be sign that the turtle is boy or not? Is this sign so specific for turtle`s males? Because smaller turtle (Kumberland) did not respond to his threat, but in the same way (minute later) he scared a stone exactly like the bigger turtle did it. And I think... does this mean that both are boys or not?
P.S. Earl is just 3.5 inches now = 9 cm and he (or she) is about one year old.
Here is how both turtles look right now (today photo)

Take a look here: http://www.redearslider.com/determining_sex.html males have very long front finger nails.

Fluttering is both used for the mating dance and their dominance dance. Both males and females will do it. Just one turtle saying to the other this is my area.

Thank you, very much, Diamondbacks4Life. Yes, it really looks like dominance dance, because Earl believes aquaterrarium his seniority I was not sure, but exactly it looks like dominance dance.
And about the finger nails... I don`t know, may be it is wise to wait a bit else. Earl is too little else for exactly determination of sex, I think. And the turtle exactly responds to name Shipa not Earl or Graf. Suddenly, I called him(her) for hissing (hissing in Russian is шипение [ship-enie]) as Shipa (sounds like word ship in English) Turtle hears this sound better, perhaps. Exactly that both know their names. I have heard that in China red ear sliders can participate in easy street show with lucky tiсkets.

Wait until they are about 4".
The females have an opening under their tail that's close to the shell.
The males have an opening under their tail that near the end of the tail.
As the male gets older, his tail gets longer and thicker.
The female has nicely trimmed front claws that grow proportionate to her back feet claws.
The male grows very long dagger-like front claws that he will use to vibrate when approaching a female. She will respond the same way. But the male will use his long claws to "tickle" the top of her head from behind so she will relax and not run away.

I'm not sure that my husband is thrilled with the idea of ​​breeding turtles He arrives in July and has not seen them else. I said him that I got little turtles. But he does not know that he will expect buying large aquaterrarium and create the second floor. But he asked his friend to help me with delivering at home small ExoTerra aquaterrarium. He just said "thank you" that I didn`t bought crocodiles without him. So, it doesnt matter for me about their sex. I was afraid that if two males are here that it means two aquaterrariums.