How To See Through an Invisibility Cloak
AMESN writes "Ways to bend light around objects and render them invisible are becoming a major field of scientific study and gaining ground. While no actual invisibility cloak exists yet, researchers are also theorizing on how to beat the perfect cloak."
Just throw a stone at it.
TFA mentions using charged particles and multiple wavelengths of EM to detect a clocked object. TFA suggests that they were measuring the actual effect on the path of the radiation its self although it should be pointed out that this is quite possibly unnecessary as high energy charged particle entering a solid material undergo an extremely high de-acceleration phase which causes charged particles to emit EM radiation. It's called Bremsstrahlung radiation and could quite possibly be detected.
Sigs are too short to say anything truly profound so read the above post instead.
Use a scrying spell, obviously.