I think it would be about the crease. If its standards are weak, or just damaged by the wind there will be a crease across the midrib and that crease will bend toward flat.
Another way you can tell is look at the ovary and how the petals are attached. A non-flat will have the same old oval ovary you're used to seeing. On a flat it's as if the bloom didn't grow a neck first, it just pops out of the ovary; the result is that flattie ovaries look dented.
I think that there are a few registered flatties that only have three beards, and there are definitely non-flatties that have at least partial beards on otherwise normal standards. So.. I wouldn't trust the beards... entirely.
--- In email@example.com, Linda Mann <lmann@...> wrote:
> How can I tell the difference between a true flattie and a regular bloom
> that just has weak midribs on standards that flop open in heat and wind
> and stay that way?
> I'm getting quite a few of those, also not keeping any of them.
> Also had one with 5 falls plus 1 standard this year, but assume that was
> just a freak of the season.
> <I usually rip these out the minute they bloom - took a pic first .
> Linda Mann east TN USA zone 7