If you can invoke Perl from the command line, you can say:

perl -MMail::Sendmail -e ''
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If it says nothing, the module is in the INC path. Otherwise, it will loudly complain "Can't locate Y.pm in @INC (@INC contains:..." the rest of the message is a list of directories where it looked for the module, but did not find it.

Warning: I don't work on windows, but I think this works there too.

Phil