How to check video grabber frame rate on iOS

Greetings
In my project (iphone 4), I do grabber->setDesiredFrameRate(my_ideal_frame_rate), I’m wondering if there is a way to check the actual grabber frame rate?

I set my_ideal_frame_rate to 24 but I start to suspect that the frame rate might not be a constant when the app is in operation at different level of computation load?

Thanks

Here is one we used a while back to check the fps of a USB camera

Thanks, it works for me.