Uses pyobjc and IOKit to check for the BootDriveIsInternalSolidState value.

Has SSD.py

#!/usr/bin/python

# Based on some nice gists by Mike Lynn (@pudquick)

import objc
from CoreFoundation import NSBundle

IOKit_bundle = NSBundle.bundleWithIdentifier_('com.apple.framework.IOKit')

functions = [("IOServiceGetMatchingService", b"II@"),
             ("IOServiceMatching", b"@*"),
             ("IORegistryEntryCreateCFProperty", b"@I@@I"),
            ]

objc.loadBundleFunctions(IOKit_bundle, globals(), functions)

def is_SSD():
    x86Platform = IOServiceGetMatchingService(0, IOServiceMatching("X86PlatformPlugin"))
    return IORegistryEntryCreateCFProperty(x86Platform, 'BootDriveIsInternalSolidState'.encode("utf-8"), None, 0)

if __name__ == '__main__':
    if is_SSD():
        print "true"
    else:
        print "false"

The "true" & "false" output from the main function is just a convention that works well with boolean Custom Facts in Addigy.