Well there are a couple of possibilities to check if there are any object, person
But one of the simplest is to use Haar-like features.
To work with Haar features (see the example FaceDetect.cpp in your directory OpenCV-2.4.3/samples/c/facedetect.cpp)
But basically works with haar files which store an abstract of the information about what is a face (any object) and what is not a face (a object)
Depending of the necessity and the power of our machine, we should different haar libraries. Even we can build special libraries for our purposes. (but my i3 with 4 GB ram took 3 days to make one haar library about cars)
I used this three
"./haarcascades/haarcascade_frontalface_alt_tree.xml" (3.5 MB) (over 500ms)
"./haarcascades/haarcascade_frontalface_alt2.xml" (0.8 MB) (over 300 ms)
"./haarcascades/haarcascade_eye.xml" (0.4 MB) (over 200 ms)
Also depends the size of the image, I used the less quality (160x120) for the first ideas will works
The code is based in the sample, so you'll find the code there, any doubt ask me.
With this exercise I finish the first part of the project based on computer vision.
We can do a lot of things more, blobs, our haar libraries, structural analyzes, movement studies, etc... but all this exercise are outside of this scope.