Is there a way to detect if a program is open from the C++ program (Windows)?

I have a C++ program that involves opening a program (let's say calculator.exe). I need to be able to test to make sure this program is open. Here's an example of what I mean.

#include <iostream.h>

int main()
{
    system("start calculator");
    if (xxxxx)
    {
        cout << "Calculator is open.\n";
    }

    cin.get();
    return 0;
}

What would I need to put in for xxxxx that tests whether the calculator is open?