std::numeric_limits::tinyness_before

#include <iostream>
#include <limits>
#include <cmath>
#include <cfenv>
int main()
{
    double denorm_max = std::nextafter(std::numeric_limits<double>::min(), 0);
    double multiplier = 1 + std::numeric_limits<double>::epsilon();

    std::feclearexcept(FE_ALL_EXCEPT);

    double result = denorm_max*multiplier; // Underflow only if tinyness_before

    if(std::fetestexcept(FE_UNDERFLOW))
        std::cout << "Underflow detected\n";
    else if (std::fetestexcept(FE_INEXACT))
        std::cout << "Inexact result detected\n";

    std::cout << std::hexfloat << denorm_max << " x " << multiplier  <<  " = "
              << result << '\n';
}