I use the following code in ex_cldr to detect if Elixir is compiling. The mechanism changed from Elixir 1.11, hence the conditional check:

  def compiling? do
    # TODO: When we depend on Elixir v1.11+ only, remove function_exported and elixir_compiler_pid
    process_alive?(:can_await_module_compilation?) ||
      process_alive?(:elixir_compiler_pid)
  end

  defp process_alive?(:can_await_module_compilation?) do
    Code.ensure_loaded?(Code) &&
      function_exported?(Code, :can_await_module_compilation?, 0) &&
      apply(Code, :can_await_module_compilation?, [])
  end

  defp process_alive?(name) do
    case Process.get(name) do
      nil -> false
      pid when is_pid(pid) -> true
    end
  end

Please do not do this:

This is relying on internal compiler behaviour. Use the suggestions from @kip instead, which uses public APIs.

That distinction doesn’t change anything he said though, you can still generate a module at runtime and compile it and for it what is happening will be “compile time”. You really should revise what you really want to achieve with this and consider other ways, this can very easily bite you in the rear in the most inopportune moment in the future.