How can I tell if this spare part is okay?

So I need new calipers on my car, to buy them new is $580 each, to get mine reconditioned is $290 each with no guarantee that they will be uncorroded enough to use. All of this is without labour etc. So i've just found a second hand set at a wreckers that is $65 each but I really don't know how good they are. How can I tell that they are okay? I understand that they can't get moisture in them or they corrode but is there any way to find out visually?