How to detect circularly polarized wave ?

"Detect" means just receive or specifically determine the polarization?

You'll receive the wave with any loop plane parallel to wave propagation direction.

If you see the anatomy of a LNA sitting at the top of the dish antenna, you will notice two short wire segments (perpendicular to each other) that act as antenna. Now you have the following possibilities:

1. None of them get any voltage- there is no signal;

2. The horizontal one gets signal but the vertical one does not: the signal is having horizontal polarization;

3. The vertical wire sees some signal but the horizontal one does not: the received signal is vertically polarized;

4. Both the wires get to see some signal: the signal is circularly polarized.

It is still possible to differentiate the LCP and RCP signals but that is another story altogether.