Check if a method exists

if ([obj respondsToSelector:@selector(methodName:withEtc:)]) {
   [obj methodName:123 withEtc:456];
}

There is also the static message instancesRespondToSelector:(SEL)selector You would call it like this:

[MyClass instancesRespondToSelector:@selector(someMethod:withParams:)]

or like this:

[[myObject class] instancesRespondToSelector:@selector(someMethod:withParams:)]

This may be useful if you would like to call one constructor or another one depending on this (I mean, before having the instance itself).

Use respondsToSelector:. From the documentation:

respondsToSelector:

Returns a Boolean value that indicates whether the receiver implements or inherits a method that can respond to a specified message.

- (BOOL)respondsToSelector:(SEL)aSelector

Parameters
aSelector - A selector that identifies a message.

Return Value
YES if the receiver implements or inherits a method that can respond to aSelector, otherwise NO.