main.m

#import <Foundation/Foundation.h>

int main (int argc, const char * argv[]) {
    NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];

    // insert code here...
	NSDictionary *immutable = [[NSDictionary alloc] init];
	NSMutableDictionary *mutable = [[NSMutableDictionary alloc] init];
	
	NSLog(@"Classnames: %@ and %@",[immutable className], [mutable className]);
	
	//test respondsToSelector for setObject:forKey:
	if([immutable respondsToSelector:@selector(setObject:forKey:)]){
		NSLog(@"immutable responds to setObject:forKey:");
	}

	if([mutable respondsToSelector:@selector(setObject:forKey:)]){
		NSLog(@"mutable responds to setObject:forKey:");
	}
	
	//test against NSMutableDictionary's class
	if([immutable isKindOfClass:[NSMutableDictionary class]]){
		NSLog(@"immutable is a mutable dict");
	}
	
	if([mutable isKindOfClass:[NSMutableDictionary class]]){
		NSLog(@"mutable is a mutable dict");
	}
    
	[pool drain];
    return 0;
}

stdout

 NSCFDictionaryTest[34961:10b] Classnames: NSCFDictionary and NSCFDictionary
 NSCFDictionaryTest[34961:10b] immutable responds to setObject:forKey:
 NSCFDictionaryTest[34961:10b] mutable responds to setObject:forKey:
 NSCFDictionaryTest[34961:10b] immutable is a mutable dict
 NSCFDictionaryTest[34961:10b] mutable is a mutable dict

successful.m

// this works, although it requires exceptions

BOOL DictIsMutable(NSDictionary *dict){
	BOOL isMutable = YES;
	@try {
		id oldValue = ([dict objectForKey:@"SSMutabilityTest"]);
		[dict setObject:@"foo" forKey:@"SSMutabilityTest"];
		//if immutable, it will throw an exception
		if(oldValue) [dict setObject:oldValue forKey:@"SSMutabilityTest"];
	}
	@catch (NSException * e) {
		isMutable = NO;
	}
	return isMutable;
}