Trying to simplify it a hair. If it has a decocker, it's alloy framed.
Well pretty much. There are odd balls out there. I know a couple members have bought surplus Israeli PCR's that have a steel frame.
See OP, just when you've got it figured out, CZ throws a curveball at you. 
CZ does seem to love to do that... -.- I was debating mentioning the SP-01 Compact that's been all the rage for the past month, but figured that wouldn't be simplifying it.
Like any good rule, there's always exceptions. That one is news to me, didn't know that they ever made something like that!
