Both of these SHOULD be marked as to the model number, but the 81 was marked in a better way, usually with "Woodsmaster 81" on the left side of the receiver; the 8 (and a sub-version, the 8A) should be marked on the barrel jacket. The model 8 was replaced in 1936 by the model 81, so if you know when it was purchased, that could be another clue.