Easier way to port packages to older Emacsen

[Radon Rosborough]

I maintain some Emacs packages that work on Emacs 25. It's been
requested that I adjust them so that they will also work on Emacs 24.
One way to do this is to install Emacs 24, try to use the packages,
and see what errors I get.

Is there a better way than this? For example, some package that scans
my code and checks if I use any variables or functions that were only
added in recent versions of Emacs.

Thanks,
Radon Rosborough