Checking if CTFE is used?

[berni via Digitalmars-d-learn]

Is there a way to check if a function is indeed executed at 
compile time or not? (Other than going through the whole 
executable binaries...)

I tried
static this()
{
  if (__ctfe) pragma(msg,"works");
  // some other stuff
}

but unfortunatley this "if" is also executed at compile time, 
when I put it into a function that is only called at runtime. 
When I try "static if" instead the compiler complains about 
"__ctfe" being not known at compile time.