40% silver Ikes will have an S mintmark and be proofs and mint strikes. And I believe that after a certain year, there are some S mintmark proofs that are clad as well.
You can easily spot the silver Ikes, in that the rims will look just like 40% silver Kennedy halves.
I own some 40%ers that I purchased, and the rims don't look anything like the clad, even more so than Kennedy 40%ers vs. clad Kennedy halves.
Jim