If they are silver colored, it's nickel plating to make them slicker for feeding and to prevent corrosion... superior to plain brass. I KNOW the Critical Defense is nickel-plated and most Winchester defense ammo is as well.
Check with a magnet if your still worried, but usually just the cheap Russian and East European stuff is steel-cased.
Wynn
USAF Retired '88, NRA Life Member. Wife USAF Retired '96
Avatar: Wynn re-enlists his wife Desiree, circa 1988 Loring AFB, ME. 42nd BMW, Heavy (SAC) B-52G's
Frédéric Bastiat’s essay, The Law: http://mises.org/books/thelaw.pdf
Thomas Jefferson said
“A government big enough to give you everything you want, is strong enough to take everything you have.”
and
"Peace is that brief glorious moment in history when everybody stands around reloading".