According to Clausager, "The cars were produced from March 1979 (car number 492071), the last Californian car was 500344 (December 1979) and the last Federal car of all was 523000 (August 1980)...."
You can spot the real LEs because they're black and say 'LE' on them! ;-) I'd imagine that without a Heritage certificate, it would be hard to tell a genuine from a good fake. Although with 6682 LEs built (Clausager), odds are if it claims to be an LE, it probably is.....
The distinguishing features are (again, from Clausager):
Black
Black or Beige trim
Silver stripe tape scheme
5-spoke cast-alloy wheels, painted dark metallic gray
Under-bumper air dam
3-spoke light allow steering wheel with leather trim
Plaque on glovebox with the MG badge and the words "Limited Edition" in gothic script
HTH!