How to determine what C++ standard is the default for a C++ compiler?

Add to max66's answer. There is no need to compile and execute the program. The same information can be grepped through the preprocessed output using:

 g++ -x c++  -E -dM -< /dev/null | grep __cplusplus

The values of the __cplusplus macro gives the value of the standard.

What about compiling and executing the following trivial program ?

#include <iostream>

int main()
 { std::cout << __cplusplus << std::endl; }

The value printed should say the version used:

• 199711 for C++98,
• 201103 for C++11
• 201402 for C++14
• 201703 for C++17

If you compile omitting the -std=c++xx flag, you should be able to detect the default version of language used.