How do I know...

Can't verify this as fact, but It is my understanding the MkII came out about 1981. That is around the time they changed from a plastic trigger to metal. Not sure what other differences there are except I read some MkII are stamped on the receiver.

Below was copied from http://forum.vintageairgunsgallery.com/feinwerkbau-fwb/feinwerkbau-serial-numbers/
Puts your rifle around 1982-83 making it a MkII.
FWB Sport 120-series (Manufactured 1973-89)
8899 -1973 Factory verified
10012 -1977
17023 -12/78
18746 -1979
34378 -09/80
34476 -09/80
39127 -1981
40003 -05/81
44001 -09/81
49992 -10/82
51631 -1983
52589 -05/83
56026 -1984
59458 -1985

Steven