Go to Admin -> System information, and read the Jira version, the versions of all the installed apps, and the "modified files" section.
Patches to Jira are mostly done by upgrading to a newer version (and apps are all done by new versions), but sometimes, a patch is done by replacing or modifying one of the installation files, and the "modified files" lists all files that have been modified away from the off-the-shelf installation. In a stock installation, the modified files will list one single file that has the location of "jira home" in it, and nothing else.
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