Opencv function runs on GPU or CPU

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It has been said in that document:

Any OpenCV function that accepts Input/OutputArray(s) also accepts UMat, even if the function has no actual (GPU) OpenCL implementation (in this case an implicit data copy can happen).

I've a program which uses opencv 3.4.1 as the main library, so I'm using many functions from it. All my images are of type cv::UMat and I've OpenCL activated and GPU from nvidia (I'm sure that my functions are running on the GPU and CPU).

My question is how can I know if a particular function is running on the GPU or CPU to perform its task?