Even ammo from the First World War is useable, as long as it's not too corroded. Usually visible corrosion (often a bluish-white crud around the neck of the casing or around the primer) means it's been exposed to moisture and it's no good. Just remember that old ammo us usually corrosive, since corrosive propellant lasts longer, you need to clean your gun the second you're done firing it.

Just toss anything rusty and make sure you clean it well afterward.