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Examine how browsers implement navigator.javaEnabled, and spec it.
http://lxr.mozilla.org/mozilla-central/source/dom/base/Navigator.cpp#611
That link will go stale when that file is changed, fwiw. A more permanent one: http://hg.mozilla.org/mozilla-central/file/85d75ed04851/dom/base/Navigator.cpp#l611
Seems to just be true if navigator.mimeTypes contains "application/x-java-vm". Should straight-forward enough.