Something nice might be if somebody released a module called issing::XS on CPAN, so that:

perl -Missing::XS your/script.pl
[download]

Would detect, for example, if JSON.pm got loaded but JSON::XS wasn't installed and print out a warning.

Obviously it would need to have a hard-coded list of modules that have XS backends, so would need to be updated occasionally. (It could also perhaps check and warn if issing::XS itself were out of date!)