You could try weighing them and see if it matches the weights given in the Red Book. I have a Morgan and a Walker half that are dull in color, and both way under weight.
Goldgrubber has a good start.If they are dull and weigh less than they should they are probably fake.
I found a cap bust half in area i was hunting and couldnt catch my breath.Then i realized that it did not read like silver and was a dull gray color.Took it to a coin show and sure enough the guy said it was German silver.Kinda like pewter but fake none the less.
Has anyone else ever found fakes