Best way to test if a file descriptor is valid?

[Noah]

I have a method that is passed an open file descriptor.
This file descriptor may have been opened from a file or
from a serial device or whatever. Is there a good way to test if 
the file descriptor is valid and available for read/write?

Currently I'm doing this:
    try:
        os.fstat(command)
    except OSError:
        raise Exception, 'Command is not a valid file descriptor.'

Which is OK I guess... Is there another way?

Yours,
Noah