LilyPad Accelerometer!! It is simply the perfect product for doing that.
First of all, it’s based on the ADXL335 MEMS accelerometer from Analog Devices, and that’s the main reason that explains its capabilities.
The ADXL335 outputs a 0V to 3V analog signal on each of the X, Y, and Z axis.
However, in order to calculate a true 'angle', you will need to convert this analog voltage to a gravity and trigonometry.
Especially if your main concern is just basic motion sensing and not tilting then this product definitely suits you.
It’s a wearable e-textile technology developed by Leah Buechley and cooperatively designed by Leah and SparkFun. Each LilyPad was creatively designed to have large connecting pads to allow them to be sewn into clothing. Various input, output, power, and sensor boards are available. They're even washable