F 122s will always:
1. Have same color F keys
2. Have stepped modifiers
3. Have a big IBM silver label
4. Have big edges
5. Have a black colored metal rear case
6. Have a label on said metal back that will actually say "Model F"
Note that there are select old (and uncommon) 122 Ms that meet 1-4, but not 5-6. So it's quite hard to confuse the two unless all one has are blurry pics of the top or something.