There's only one way I know of to tell an AR-15 from an M-16.
   Pick it up, hold it about a foot from your face and look closely
   at the saftey lever.  If it has two positions, its an AR-15, if
   it has three, its an M-16.  There are numerous internal differences
   as well, but since one would have to field strip the weapon to see
   them, they are not valid in this discussion.  So, in conclusion,
   there is very little external differences to distinguish an AR-15
   from an M-16 except at close (very close) range.

   David Bixler
   Auburn University