How to determine if returned address is fastram

[mfro]

I'd say there is no reliable way to programatically achieve what you want.
"FastRAM" might be faster or slower than ST-RAM or even the exact same speed (as on the FireBee with FireTOS or an ST extended beyond 4MB).
With the Maddalt() system call, you can add VME RAM to FastRAM (which would be extremely slow) or even "virtual" RAM that might be required to be swapped in first.
The only "one fits all" solution to your problem is probably to let the user (who should know) configure RAM areas.