See from top to both Led . You will see black spot on one led. On another one you will see grey spot. The led consisting of black spot is the iR receiver (ir transister). Another one is ir emmiter. If you see only white spot then it is normal led.
Ir sensor work on property of ir light that ir light absorbed by black surface and totally reflected by white surface. Ir led emit ir light which is reflected by surface is absorbed by ir transistor(ir receiver). Ir receiver work on property that when light fall on an transister on his base emitter resion than its CE current increases due to emition of electron.