so far as I know, all 5800's and 5800 Ultra's have FX-Flow. The main difference is check the specs, because non-ultra have a core of 400mhz, while Ultra has 500mhz. If no specs are listed, then I'm afraid there is no way to know the difference strictly by physical appearance.
Only 5600 and 5200 donot come with FX-Flow
Athlon64 3200 Clawhammer / Asus K8N-E Deluxe / 1GB Crucial DDR400 / Geforce 6600GT 128MB AGP / Windows XP Pro