I used to leave mine on constantly. On a warm humid day condensation would form on the outside of the windscreen, just above the vents. That was the best check it was working.
I tried fuel consumption figures with air con and without, and the difference was lost in the scatter of figures. Nothing that was worth the worrying about.
The car (Volvo S40) had 48K miles and was 6 year old when I bought it. I sold it 10 years later with 131K miles on the clock and the air con worked flawlessly, summer and winter, for the entire time I had it (other than the couple of top ups I tried with it off).
The car also had a sunroof, but it was kept shut, with the blind across, for the entire time I had it. Personally, I think a sunroof is just a waste of money. On a car without air con it can be nice to tilt the sunroof on a warm day, but opening it just lets the sun bake the top of your head!