Well, I think most people do it by some sort of "trial and error", the tool of choise is Matt Godbolt's Compiler Explorer [1]

I sometimes listen to the CppCast Podcast and one of the Hosts, Jason Turner, likes to write code in modern C++ for some vintage computers. He tests code bits in Compiler Explorer for that. He also has a YouTube Channel where he does that [2]

[1] https://gcc.godbolt.org/

[2] https://www.youtube.com/user/lefticus1