Easy test for A/C - I do it all the time....

While at cruising speed (either highway or city with cruise control on and A/C on) do this:

1. Look at your current real-time MPG and remember it.
2. Press the A/C button to turn the compressor off.
3. Look at the current real-time MPG again. Let it settle for a few seconds.
4. Press the A/C button back on.
5. Look at the real-time MPG again.

I usually see a 5-12 MPG drop with A/C on, depending on the speed and the road levels.