All objects that inherit from NSObject know these three methods:
isKindOfClass: returns whether an object is that kind of class (inheritance included)
isMemberOfClass: returns whether an object is that kind of class (no inheritance)
respondsToSelector: returns whether an object responds to a given method

Class testing methods take a Class
You get a Class by sending the class method class to a class :)
if ([obj isKindOfClass:[NSString class]]) {
NSString *s = [(NSString *)obj stringByAppendingString:@”xyzzy”];
}

Method testing methods take a selector (SEL)
Special @selector() directive turns the name of a method into a selector
if ([obj respondsToSelector:@selector(shoot)]) {
[obj shoot];
} else if ([obj respondsToSelector:@selector(shootAt:)]) {
[obj shootAt:target];
}
SEL is the Objective-C “type” for a selector
SEL shootSelector = @selector(shoot);
SEL shootAtSelector = @selector(shootAt:);
SEL moveToSelector = @selector(moveTo:withPenColor:);