How to tell if `constexpr` is evaluated at compile time (without manual inspection)

Is there a standard way to find out what the compiler does to constexpr functions?

(Side note: For debug, every constexpr function is deferred to runtime by default. Why is this sensible? Is there a way to influence this?)

For release it depends on the context. Obviously, for small test settings you can easily inspect the generated machine code, but this cannot be the way to go for a real project.

My current 'workaround' (VC++) is to break somewhere, go to my constexpr function and (try to) inspect the disassembly. If none is there, I conclude that it was all done at compile time. But it is not 100% reliable this way. (Optimization, etc.) Only the other way around is certain: If I do find disassembly (and can even break there), I know that it was NOT done at compile time.