If that's the only thing that is necessary I would use android gestures. You can find examples and more info over here. You just have to create an oval, circle, triangle gesture and Android will recognize it for you. It's really easy, productive and does exactly what you want. In the link is a step-by-step procedure of how to do this.
You can do it also by saving all the x/y coordinates of the drawn shape and then compare it using circle/oval equations.
I would go for the first solution if you don't have any additional requirements.
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You can copy the gestures from the emulator to your computer with
./adb pull /sdcard/gestures ~/test
You have to put them in de res/raw folder to use them in your application.
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adb commands: click here