What are your techniques to find out how CPU intensive are some parts of your code?

Sometimes you have more than one solution ideas for one problem - maybe all of them work. Which one to use? Maybe that one, what lesser burns the CPU/GPU and saves battery time - I think users take care of it. If I feel the iPhone gets hotter while playing some special games - I get a strange feeling.

I would like to test differences of CPU cost on real devices: iOS and Android
I'm not interested to test it in the simulator - as I think it's not the same.

There are some more questions I could answer using a real test:
How much more costs the change of some physics engine parameters?
How much physics objects more are in the budget?

Thanks!