Unit testing of a function that doesn't return any value

I am doing unit testing in c++ using cppunit. I have seen some examples over internet that are showing testing a function that return some value. I have some functions in my project that do not return any value but do some calculations inside. Here I want to test the calculations inside those functions whether calculations are being done correctly or not, how can I do it? For example I have a function

void Flash::SerializeToBytes(Buffer &buffer) const
{
    // calculations
}