I've seen another work around to detect whether or not a file is being
byte-compiled in use-package
<https://github.com/jwiegley/use-package/blob/master/use-package-ensure.el#L192-L197>:
(bound-and-true-p byte-compile-current-file).

It seems to work, as illustrated by this example:

(message
 (eval-when-compile
   (if (bound-and-true-p byte-compile-current-file)
       "I'm being byte-compiled!"
     "I'm being evaluated :(")))

When I byte compile that and then evaluate

(progn
  (load-file "~/byte-compile-test.elc")
  (load-file "~/byte-compile-test.el"))

It prints out

Loading /Users/jake.waksbaum/byte-compile-test.elc...
I’m being byte-compiled!
Loading /Users/jake.waksbaum/byte-compile-test.elc...done
Loading /Users/jake.waksbaum/byte-compile-test.el (source)...
I’m being evaluated :(
Loading /Users/jake.waksbaum/byte-compile-test.el (source)...done
t