[OS X TeX] Testing for fontspec package

[firstian at rcn.com]

I have a tex file that needs to be compiled for both xelatex and regular latex (both 
use pdflatex to generate pdf directly). The tex file currently uses fontspec, which 
the regular latex doesn't like. Is there a way to test for whether fontspec is installed 
in the tex file itself, so it can work for both without modifications?

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