Maybe something like:

#include <fstream>
#include <iostream>
#include <string>

int main() {
  std::ifstream status("/proc/self/status");
  std::string line;

  while (std::getline(status, line)) {
    if (line.find("CapEff") == 0) {
      std::cout << line << std::endl;
    }
  }

  return 0;
}
EOF
$ sudo ./captest
CapEff: 0000003fffffffff
$ sudo ./minijail0 -T static -c 1fffffffff ./captest
Can't run statically-linked binaries with capabilities (-c) \
  without also setting ambient capabilities. Try passing --ambient
$ sudo ./minijail0 -T static -c 1fffffffff --ambient ./captest
CapEff: 0000001fffffffff