Fell ponies and Dales ponies are very similar - both from the same county, just different sides of the Pennines.
The Fell is smaller and you actually see it in the other colours allowed by the breed society - I have seen the usual black as well as some bays and also a very nice grey. The head is very pony-like (quite small and neat) and they are allowed to have a small star and a sock on a hind leg.
The Dales is taller, rarely found in the other colours allowed by the breed society (I've only ever seen them in black) and has a very horse-like head (a bit longer in the nose than the Fell.) They are also allowed the same markings.
The breed standards for both ponies say they have pony-like heads, but if you compare the two, the Fell's head is a little neater and more pony like and that of the Dales is a similar shape to that of the Friesian.
The Dales tend to have silky feather and the Fell tends to have more of a light feather
http://www.fellponysociety.org/
http://www.dalespony.org/
Fell - http://www.fellponysociety.org/photo_gallery.htm
Dales - http://www.dalespony.org/Gallery.html
EDIT
see arr harr - we use the ribbons to tell the difference too! Must be a life saver for the judges that usually judge other breeds but have been pulled in to judge the hairies.
I read somewhere that the ribbons were originally used as when showing first began with the Dales ponies, they had docked tails with enough hair to protect the tail in severe weather and the owners felt they ought to do something with their tails to neaten them up and make the ponies stand out.
Why they have the ribbons is somewhere in here: http://www.hamletshouse.co.uk/showing/showingguide.htm