Use magereport.com to scan your site. They will tell you what patches are installed and what patches still need to be installed
Other option If you use any type of version control, look back at your project history and see if you find the patches there.
Also, you can try applying the patch (on a staging environment) and see if the server throws an error that it cannot apply patch, which might mean that it was patched already.
Finally, each patch file shows exactly what's being patched, you can just open any of the files listed in the patch and check if the patched code is in there.