I think that most feminine words end in ة. So mostly you can guess it is feminine if it ends in ة and masculine otherwise. There are exceptions like بنت (daughter) and أم (mother). Fortunately in these cases it is easy to work out because these words actually refer to female people.
The "Saaluun" is a room (bigger than the other rooms) where we welcome and sit with visitors.
For the other rooms we say "ghurfa" like : ghurfat annaum غرفة النَّوْم = bedroom